feat(backend): implement gemini quota simulation and rate limiting

- feat: add local quota tracking for gemini tiers (Legacy/Pro/Ultra)
- feat: implement PreCheckUsage in RateLimitService
- feat: align gemini daily reset window with PST
- fix: sticky session fallback logic

deploy/docker-compose.yml

@@ -90,6 +90,7 @@ services:
       - GEMINI_OAUTH_CLIENT_ID=${GEMINI_OAUTH_CLIENT_ID:-}
       - GEMINI_OAUTH_CLIENT_SECRET=${GEMINI_OAUTH_CLIENT_SECRET:-}
       - GEMINI_OAUTH_SCOPES=${GEMINI_OAUTH_SCOPES:-}
+      - GEMINI_QUOTA_POLICY=${GEMINI_QUOTA_POLICY:-}
     depends_on:
       postgres:
         condition: service_healthy