Harden scaffold (reviewer followups)

- main.ts awaits DatabaseInitService.init() before app.listen(), ensuring
  migrations + seed run before the HTTP server accepts traffic.
- AppModule now uses NestModule with consumer.apply() no longer needed for
  CSRF (registered globally via app.use after body parsers in main.ts).
- JwtAuthGuard extended from AuthGuard('jwt') so protected endpoints
  actually validate the bearer token.
- Admin controller now uses Zod-validated DTOs for body/path/query:
  createUser, updateUserRole, userIdParam, listUsersQuery; with @Public
  / @Roles decorators and AdminGuard applied.
- Multer upload module + controller (POST /api/v1/uploads/category-icon
  and /challenge-file) with safe-filename strategy, configured
  UPLOAD_SIZE_LIMIT, admin-only via AdminGuard, served via /uploads
  static handler.
- ThemeLoaderService now requires all 10 canonical theme ids at startup,
  backfilling missing themes from built-ins with a warning; validates the
  configured themeKey setting and falls back to 'classic' if invalid;
  gracefully tolerates missing setting table during early boot.
- Test suite expanded to 76 tests / 17 suites; new specs:
  admin-validation, uploads, theme-required, database-init.

tests/backend/bootstrap.integration.spec.ts

@@ -8,6 +8,7 @@ import { HttpAdapterHost } from '@nestjs/core';
 import { AppModule } from '../../backend/src/app.module';
 import { GlobalExceptionFilter } from '../../backend/src/common/filters/global-exception.filter';
 import { csrfClient, primeCsrf } from './csrf-client';
+import { initDb } from './db-helper';
 
 describe('Bootstrap integration', () => {
   let app: INestApplication;
@@ -19,6 +20,7 @@ describe('Bootstrap integration', () => {
     const httpAdapterHost = app.get(HttpAdapterHost);
     app.useGlobalFilters(new GlobalExceptionFilter(httpAdapterHost));
     await app.init();
+    await initDb(app);
   });
 
   afterAll(async () => {