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.

backend/package.json

@@ -24,6 +24,7 @@
     "class-validator": "^0.14.1",
     "cookie-parser": "^1.4.6",
     "helmet": "^7.1.0",
+    "multer": "^2.0.2",
     "passport": "^0.7.0",
     "passport-jwt": "^4.0.1",
     "reflect-metadata": "^0.2.2",
@@ -37,6 +38,7 @@
     "@types/better-sqlite3": "^7.6.10",
     "@types/cookie-parser": "^1.4.7",
     "@types/express": "^4.17.21",
+    "@types/multer": "^1.4.12",
     "@types/passport-jwt": "^4.0.1",
     "@types/supertest": "^6.0.2",
     "@types/uuid": "^9.0.8",