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/db-helper.ts

@@ -0,0 +1,11 @@
+import { INestApplication } from '@nestjs/common';
+import { DatabaseInitService } from '../../backend/src/database/database-init.service';
+
+/**
+ * Await database initialization (migrations + seed verification) on the
+ * given Nest app. Without this, integration tests that hit controllers
+ * will hit a fresh `:memory:` SQLite database with no tables.
+ */
+export async function initDb(app: INestApplication): Promise<void> {
+  await app.get<DatabaseInitService>(DatabaseInitService).init();
+}