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)

backend/app/models.py

@@ -283,3 +283,8 @@ class MobileBuildArtifact(Base):
     sha256 = Column(String(64), nullable=False)
     size = Column(Integer, nullable=False)
     created_at = Column(DateTime, nullable=False, default=datetime.utcnow)
+
+
+# Import user_models so that User, Role, UserRole are registered on Base.metadata
+# before SQLAlchemy validates cross-table foreign keys (e.g. mobile_build_requests.created_by_id -> users.id).
+from app.user_models import *  # noqa: F401,F403