feat(ssr): spider-only SSR — serve CSR shell for regular users

Add spider detection gate to SSR server so that search engine
crawlers get full server-side rendering with fresh API data,
while regular users receive the static CSR app shell from disk.

- backend/app/spider_detection.py: Python spider detection module
- spider_detection.ts: TypeScript mirror of spider detection patterns
- server.ts: SSR gate — isSpider() check before dispatching to
  Angular SSR engine; non-spiders get static index.html
- server.spec.ts: Added spider detection unit tests
- tsconfig.app.json: Include spider_detection.ts in compilation
- backend/app/models.py: Fix SQLAlchemy FK resolution (user_models import)

tsconfig.app.json

@@ -9,7 +9,8 @@
   },
   "include": [
     "src/**/*.ts",
-    "server.ts"
+    "server.ts",
+    "spider_detection.ts"
   ],
   "exclude": [
     "src/**/*.spec.ts"