Go-live hardening: server_tokens, resource limits, pinned images, CVE fixes

- Add server_tokens off to nginx (suppress version leakage)
- Add deploy.resources.limits to all containers (db: 512M, api: 256M, nginx: 64M)
- Pin image tags: mysql:8.0 → 8.0.45, nginx:alpine → 1.29.6-alpine
- Fix CVEs: cryptography 43.0.3 → 46.0.5 (HIGH), python-jose 3.3.0 → 3.4.0 (CRITICAL)
- Add .limit(500) to GET /api/flock and GET /api/admin/users

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/admin.py

@@ -18,7 +18,7 @@ def list_users(
     _: User = Depends(get_current_admin),
     db: Session = Depends(get_db),
 ):
-    return db.scalars(select(User).order_by(User.created_at)).all()
+    return db.scalars(select(User).order_by(User.created_at).limit(500)).all()
 
 
 @router.post("/users", response_model=UserOut, status_code=201)