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/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"
   }
 }