feat(shifts): add Shift CRUD API with sign-up/cancel and capacity management

- ShiftService with 7 methods: list, detail, create, update, delete, signup, cancel
- 5 DTOs: ShiftListDto, ShiftDetailDto, CreateShiftRequest, UpdateShiftRequest, ShiftSignupDto
- Minimal API endpoints: GET /api/shifts, GET /api/shifts/{id}, POST, PUT, DELETE, POST /signup, DELETE /signup
- Capacity validation: sign-up rejected when full → 409 Conflict
- Past shift blocking: cannot sign up for past shifts → 422 Unprocessable
- Duplicate signup prevention: check existing before create → 409 Conflict
- Concurrency: 2-attempt retry loop for last-slot race conditions
- Authorization: POST/PUT (Manager+), DELETE (Admin), signup/cancel (Member+)
- Test infrastructure: Added X-Test-UserId header support for member ID injection
- 13 TDD integration tests: CRUD, sign-up, capacity, past shift, concurrency
- Build: 0 errors (6 BouncyCastle warnings expected)

Task 15 complete. Wave 3: 3/5 tasks done.

backend/WorkClub.Application/Shifts/DTOs/ShiftDetailDto.cs

@@ -0,0 +1,22 @@
+namespace WorkClub.Application.Shifts.DTOs;
+
+public record ShiftDetailDto(
+    Guid Id,
+    string Title,
+    string? Description,
+    string? Location,
+    DateTimeOffset StartTime,
+    DateTimeOffset EndTime,
+    int Capacity,
+    List<ShiftSignupDto> Signups,
+    Guid ClubId,
+    Guid CreatedById,
+    DateTimeOffset CreatedAt,
+    DateTimeOffset UpdatedAt
+);
+
+public record ShiftSignupDto(
+    Guid Id,
+    Guid MemberId,
+    DateTimeOffset SignedUpAt
+);