feat(auth): add Keycloak JWT authentication and role-based authorization

- Configure JWT Bearer authentication with Keycloak realm integration
- Create ClubRoleClaimsTransformation to parse 'clubs' claim and add ASP.NET roles
- Add authorization policies: RequireAdmin, RequireManager, RequireMember, RequireViewer
- Add health check endpoints (/health/live, /health/ready, /health/startup)
- Add integration tests for authorization (TDD approach - tests written first)
- Configure middleware order: Authentication → MultiTenant → Authorization
- Add Keycloak configuration to appsettings.Development.json
- Add AspNetCore.HealthChecks.NpgSql v9.0.0 package

TDD Verification:
- Tests initially FAILED (expected before implementation) ✓
- Implementation complete but blocked by Task 8 Infrastructure errors
- Cannot verify tests PASS until Finbuckle.MultiTenant types resolve

Security Notes:
- RequireHttpsMetadata=false for dev only (MUST be true in production)
- Claims transformation maps Keycloak roles (lowercase) to ASP.NET roles (PascalCase)
- Health endpoints are public by default (no authentication required)

Blockers:
- Infrastructure project has Finbuckle.MultiTenant type resolution errors from Task 8
- Tests cannot execute until TenantProvider compilation errors are fixed

backend/WorkClub.Api/Auth/ClubRoleClaimsTransformation.cs

@@ -0,0 +1,72 @@
+using System.Security.Claims;
+using System.Text.Json;
+using Microsoft.AspNetCore.Authentication;
+
+namespace WorkClub.Api.Auth;
+
+public class ClubRoleClaimsTransformation : IClaimsTransformation
+{
+    private readonly IHttpContextAccessor _httpContextAccessor;
+
+    public ClubRoleClaimsTransformation(IHttpContextAccessor httpContextAccessor)
+    {
+        _httpContextAccessor = httpContextAccessor;
+    }
+
+    public Task<ClaimsPrincipal> TransformAsync(ClaimsPrincipal principal)
+    {
+        if (principal.Identity is not ClaimsIdentity identity || !identity.IsAuthenticated)
+        {
+            return Task.FromResult(principal);
+        }
+
+        var clubsClaim = principal.FindFirst("clubs")?.Value;
+        if (string.IsNullOrEmpty(clubsClaim))
+        {
+            return Task.FromResult(principal);
+        }
+
+        Dictionary<string, string>? clubsDict;
+        try
+        {
+            clubsDict = JsonSerializer.Deserialize<Dictionary<string, string>>(clubsClaim);
+        }
+        catch (JsonException)
+        {
+            return Task.FromResult(principal);
+        }
+
+        if (clubsDict == null || clubsDict.Count == 0)
+        {
+            return Task.FromResult(principal);
+        }
+
+        var tenantId = _httpContextAccessor.HttpContext?.Request.Headers["X-Tenant-Id"].FirstOrDefault();
+        if (string.IsNullOrEmpty(tenantId))
+        {
+            return Task.FromResult(principal);
+        }
+
+        if (!clubsDict.TryGetValue(tenantId, out var clubRole))
+        {
+            return Task.FromResult(principal);
+        }
+
+        var mappedRole = MapClubRoleToAspNetRole(clubRole);
+        identity.AddClaim(new Claim(ClaimTypes.Role, mappedRole));
+
+        return Task.FromResult(principal);
+    }
+
+    private static string MapClubRoleToAspNetRole(string clubRole)
+    {
+        return clubRole.ToLowerInvariant() switch
+        {
+            "admin" => "Admin",
+            "manager" => "Manager",
+            "member" => "Member",
+            "viewer" => "Viewer",
+            _ => "Viewer"
+        };
+    }
+}

backend/WorkClub.Api/WorkClub.Api.csproj

@@ -7,8 +7,12 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="AspNetCore.HealthChecks.NpgSql" Version="9.0.0" />
+    <PackageReference Include="Finbuckle.MultiTenant" Version="10.0.3" />
+    <PackageReference Include="Finbuckle.MultiTenant.AspNetCore" Version="10.0.3" />
     <PackageReference Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="10.0.0" />
     <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="10.0.0" />
+    <PackageReference Include="Microsoft.EntityFrameworkCore" Version="10.0.3" />
   </ItemGroup>
 
   <ItemGroup>

backend/WorkClub.Api/appsettings.Development.json

@@ -4,5 +4,12 @@
       "Default": "Information",
       "Microsoft.AspNetCore": "Warning"
     }
+  },
+  "ConnectionStrings": {
+    "DefaultConnection": "Host=localhost;Port=5432;Database=workclub;Username=app;Password=apppass"
+  },
+  "Keycloak": {
+    "Authority": "http://localhost:8080/realms/workclub",
+    "Audience": "workclub-api"
   }
 }