fix: exempt /api/clubs/me from tenant validation

- Add path exemption in TenantValidationMiddleware for /api/clubs/me
- Change authorization policy from RequireMember to RequireViewer
- Fix KEYCLOAK_CLIENT_ID in docker-compose.yml (workclub-app)
- Resolves frontend chicken-and-egg problem for club discovery

Verified:
- /api/clubs/me returns 200 OK without X-Tenant-Id header
- /api/tasks still requires X-Tenant-Id (400 Bad Request)
- Other endpoints unaffected

backend/WorkClub.Api/Endpoints/Clubs/ClubEndpoints.cs

@@ -11,7 +11,7 @@ public static class ClubEndpoints
         var group = app.MapGroup("/api/clubs");
 
         group.MapGet("/me", GetMyClubs)
-            .RequireAuthorization("RequireMember")
+            .RequireAuthorization("RequireViewer")
             .WithName("GetMyClubs");
 
         group.MapGet("/current", GetCurrentClub)