fix(backend): resolve shift signup by looking up Member via ExternalUserId

The signup/cancel endpoints were passing the Keycloak sub claim (external UUID)
directly as MemberId, but ShiftSignup.MemberId references the internal Member.Id.
Now ShiftService resolves ExternalUserId to the internal Member.Id before creating
the signup record. Integration tests updated to seed proper Member entities.

backend/WorkClub.Tests.Integration/Shifts/ShiftCrudTests.cs

@@ -3,6 +3,7 @@ using System.Net.Http.Json;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.DependencyInjection;
 using WorkClub.Domain.Entities;
+using WorkClub.Domain.Enums;
 using WorkClub.Infrastructure.Data;
 using WorkClub.Tests.Integration.Infrastructure;
 using Xunit;
@@ -23,9 +24,60 @@ public class ShiftCrudTests : IntegrationTestBase
         // Clean up existing test data
         context.ShiftSignups.RemoveRange(context.ShiftSignups);
         context.Shifts.RemoveRange(context.Shifts);
+        context.Members.RemoveRange(context.Members);
+        context.Clubs.RemoveRange(context.Clubs);
         await context.SaveChangesAsync();
     }
 
+    private async Task<(Guid clubId, Guid memberId, string externalUserId)> SeedMemberAsync(
+        string tenantId,
+        string email,
+        string? externalUserId = null,
+        ClubRole role = ClubRole.Member)
+    {
+        externalUserId ??= Guid.NewGuid().ToString();
+        var clubId = Guid.NewGuid();
+        var memberId = Guid.NewGuid();
+        var now = DateTimeOffset.UtcNow;
+
+        using var scope = Factory.Services.CreateScope();
+        var context = scope.ServiceProvider.GetRequiredService<AppDbContext>();
+
+        var existingClub = await context.Clubs.FirstOrDefaultAsync(c => c.TenantId == tenantId);
+        if (existingClub != null)
+        {
+            clubId = existingClub.Id;
+        }
+        else
+        {
+            context.Clubs.Add(new Club
+            {
+                Id = clubId,
+                TenantId = tenantId,
+                Name = "Test Club",
+                SportType = SportType.Tennis,
+                CreatedAt = now,
+                UpdatedAt = now
+            });
+        }
+
+        context.Members.Add(new Member
+        {
+            Id = memberId,
+            TenantId = tenantId,
+            ExternalUserId = externalUserId,
+            DisplayName = email.Split('@')[0],
+            Email = email,
+            Role = role,
+            ClubId = clubId,
+            CreatedAt = now,
+            UpdatedAt = now
+        });
+
+        await context.SaveChangesAsync();
+        return (clubId, memberId, externalUserId);
+    }
+
     [Fact]
     public async Task CreateShift_AsManager_ReturnsCreated()
     {
@@ -343,8 +395,8 @@ public class ShiftCrudTests : IntegrationTestBase
     public async Task SignUpForShift_WithCapacity_ReturnsOk()
     {
         // Arrange
+        var (clubId, memberId, externalUserId) = await SeedMemberAsync("tenant1", "member@test.com");
         var shiftId = Guid.NewGuid();
-        var clubId = Guid.NewGuid();
         var createdBy = Guid.NewGuid();
         var now = DateTimeOffset.UtcNow;
 
@@ -370,7 +422,7 @@ public class ShiftCrudTests : IntegrationTestBase
         }
 
         SetTenant("tenant1");
-        AuthenticateAs("member@test.com", new Dictionary<string, string> { ["tenant1"] = "Member" });
+        AuthenticateAs("member@test.com", new Dictionary<string, string> { ["tenant1"] = "Member" }, externalUserId);
 
         // Act
         var response = await Client.PostAsync($"/api/shifts/{shiftId}/signup", null);
@@ -384,6 +436,7 @@ public class ShiftCrudTests : IntegrationTestBase
             var context = scope.ServiceProvider.GetRequiredService<AppDbContext>();
             var signups = await context.ShiftSignups.Where(ss => ss.ShiftId == shiftId).ToListAsync();
             Assert.Single(signups);
+            Assert.Equal(memberId, signups[0].MemberId);
         }
     }
 
@@ -391,11 +444,14 @@ public class ShiftCrudTests : IntegrationTestBase
     public async Task SignUpForShift_WhenFull_ReturnsConflict()
     {
         // Arrange
+        var (clubId, _, externalUserId) = await SeedMemberAsync("tenant1", "member@test.com");
         var shiftId = Guid.NewGuid();
-        var clubId = Guid.NewGuid();
         var createdBy = Guid.NewGuid();
         var now = DateTimeOffset.UtcNow;
 
+        // Seed a different member to fill the single slot
+        var (_, fillerMemberId, _) = await SeedMemberAsync("tenant1", "filler@test.com");
+
         using (var scope = Factory.Services.CreateScope())
         {
             var context = scope.ServiceProvider.GetRequiredService<AppDbContext>();
@@ -420,7 +476,7 @@ public class ShiftCrudTests : IntegrationTestBase
                 Id = Guid.NewGuid(),
                 TenantId = "tenant1",
                 ShiftId = shiftId,
-                MemberId = Guid.NewGuid(),
+                MemberId = fillerMemberId,
                 SignedUpAt = now
             });
 
@@ -428,7 +484,7 @@ public class ShiftCrudTests : IntegrationTestBase
         }
 
         SetTenant("tenant1");
-        AuthenticateAs("member@test.com", new Dictionary<string, string> { ["tenant1"] = "Member" });
+        AuthenticateAs("member@test.com", new Dictionary<string, string> { ["tenant1"] = "Member" }, externalUserId);
 
         // Act
         var response = await Client.PostAsync($"/api/shifts/{shiftId}/signup", null);
@@ -441,8 +497,8 @@ public class ShiftCrudTests : IntegrationTestBase
     public async Task SignUpForShift_ForPastShift_ReturnsUnprocessableEntity()
     {
         // Arrange
+        var (clubId, _, externalUserId) = await SeedMemberAsync("tenant1", "member@test.com");
         var shiftId = Guid.NewGuid();
-        var clubId = Guid.NewGuid();
         var createdBy = Guid.NewGuid();
         var now = DateTimeOffset.UtcNow;
 
@@ -455,7 +511,7 @@ public class ShiftCrudTests : IntegrationTestBase
                 Id = shiftId,
                 TenantId = "tenant1",
                 Title = "Past Shift",
-                StartTime = now.AddHours(-2),  // Past shift
+                StartTime = now.AddHours(-2),
                 EndTime = now.AddHours(-1),
                 Capacity = 5,
                 ClubId = clubId,
@@ -468,7 +524,7 @@ public class ShiftCrudTests : IntegrationTestBase
         }
 
         SetTenant("tenant1");
-        AuthenticateAs("member@test.com", new Dictionary<string, string> { ["tenant1"] = "Member" });
+        AuthenticateAs("member@test.com", new Dictionary<string, string> { ["tenant1"] = "Member" }, externalUserId);
 
         // Act
         var response = await Client.PostAsync($"/api/shifts/{shiftId}/signup", null);
@@ -481,10 +537,9 @@ public class ShiftCrudTests : IntegrationTestBase
     public async Task SignUpForShift_Duplicate_ReturnsConflict()
     {
         // Arrange
+        var (clubId, memberId, externalUserId) = await SeedMemberAsync("tenant1", "member@test.com");
         var shiftId = Guid.NewGuid();
-        var clubId = Guid.NewGuid();
         var createdBy = Guid.NewGuid();
-        var memberId = Guid.Parse("00000000-0000-0000-0000-000000000001"); // Fixed member ID
         var now = DateTimeOffset.UtcNow;
 
         using (var scope = Factory.Services.CreateScope())
@@ -505,7 +560,6 @@ public class ShiftCrudTests : IntegrationTestBase
                 UpdatedAt = now
             });
 
-            // Add existing signup
             context.ShiftSignups.Add(new ShiftSignup
             {
                 Id = Guid.NewGuid(),
@@ -519,7 +573,7 @@ public class ShiftCrudTests : IntegrationTestBase
         }
 
         SetTenant("tenant1");
-        AuthenticateAs("member@test.com", new Dictionary<string, string> { ["tenant1"] = "Member" }, memberId.ToString());
+        AuthenticateAs("member@test.com", new Dictionary<string, string> { ["tenant1"] = "Member" }, externalUserId);
 
         // Act
         var response = await Client.PostAsync($"/api/shifts/{shiftId}/signup", null);
@@ -532,10 +586,9 @@ public class ShiftCrudTests : IntegrationTestBase
     public async Task CancelSignup_BeforeShift_ReturnsOk()
     {
         // Arrange
+        var (clubId, memberId, externalUserId) = await SeedMemberAsync("tenant1", "member@test.com");
         var shiftId = Guid.NewGuid();
-        var clubId = Guid.NewGuid();
         var createdBy = Guid.NewGuid();
-        var memberId = Guid.Parse("00000000-0000-0000-0000-000000000001");
         var now = DateTimeOffset.UtcNow;
 
         using (var scope = Factory.Services.CreateScope())
@@ -569,7 +622,7 @@ public class ShiftCrudTests : IntegrationTestBase
         }
 
         SetTenant("tenant1");
-        AuthenticateAs("member@test.com", new Dictionary<string, string> { ["tenant1"] = "Member" }, memberId.ToString());
+        AuthenticateAs("member@test.com", new Dictionary<string, string> { ["tenant1"] = "Member" }, externalUserId);
 
         // Act
         var response = await Client.DeleteAsync($"/api/shifts/{shiftId}/signup");
@@ -577,7 +630,6 @@ public class ShiftCrudTests : IntegrationTestBase
         // Assert
         Assert.Equal(HttpStatusCode.OK, response.StatusCode);
 
-        // Verify signup was deleted
         using (var scope = Factory.Services.CreateScope())
         {
             var context = scope.ServiceProvider.GetRequiredService<AppDbContext>();
@@ -590,8 +642,11 @@ public class ShiftCrudTests : IntegrationTestBase
     public async Task SignUpForShift_ConcurrentLastSlot_HandlesRaceCondition()
     {
         // Arrange
+        var (clubId, fillerMemberId, _) = await SeedMemberAsync("tenant1", "filler@test.com");
+        var (_, _, externalUserId1) = await SeedMemberAsync("tenant1", "member1@test.com");
+        var (_, _, externalUserId2) = await SeedMemberAsync("tenant1", "member2@test.com");
+
         var shiftId = Guid.NewGuid();
-        var clubId = Guid.NewGuid();
         var createdBy = Guid.NewGuid();
         var now = DateTimeOffset.UtcNow;
 
@@ -613,13 +668,12 @@ public class ShiftCrudTests : IntegrationTestBase
                 UpdatedAt = now
             });
 
-            // Add one signup (leaving one slot)
             context.ShiftSignups.Add(new ShiftSignup
             {
                 Id = Guid.NewGuid(),
                 TenantId = "tenant1",
                 ShiftId = shiftId,
-                MemberId = Guid.NewGuid(),
+                MemberId = fillerMemberId,
                 SignedUpAt = now
             });
 
@@ -628,24 +682,20 @@ public class ShiftCrudTests : IntegrationTestBase
 
         SetTenant("tenant1");
 
-        // Act - Simulate two concurrent requests
-        var member1 = Guid.NewGuid();
-        var member2 = Guid.NewGuid();
-
-        AuthenticateAs("member1@test.com", new Dictionary<string, string> { ["tenant1"] = "Member" }, member1.ToString());
+        // Act
+        AuthenticateAs("member1@test.com", new Dictionary<string, string> { ["tenant1"] = "Member" }, externalUserId1);
         var response1Task = Client.PostAsync($"/api/shifts/{shiftId}/signup", null);
 
-        AuthenticateAs("member2@test.com", new Dictionary<string, string> { ["tenant1"] = "Member" }, member2.ToString());
+        AuthenticateAs("member2@test.com", new Dictionary<string, string> { ["tenant1"] = "Member" }, externalUserId2);
         var response2Task = Client.PostAsync($"/api/shifts/{shiftId}/signup", null);
 
         var responses = await Task.WhenAll(response1Task, response2Task);
 
-        // Assert - One should succeed (200), one should fail (409)
+        // Assert
         var statuses = responses.Select(r => r.StatusCode).OrderBy(s => s).ToList();
         Assert.Contains(HttpStatusCode.OK, statuses);
         Assert.Contains(HttpStatusCode.Conflict, statuses);
 
-        // Verify only 2 total signups exist (capacity limit enforced)
         using (var scope = Factory.Services.CreateScope())
         {
             var context = scope.ServiceProvider.GetRequiredService<AppDbContext>();