Add Google Analytics 4 with cookie consent integration

- Add GA4 measurement ID to config (default: G-J0PCVT14NY)
- Add /api/config endpoint to expose GA ID to frontend
- Update cookie consent with Analytics category (opt-in)
- Load GA4 only after user consents to analytics cookies
- Update CSP to allow Google Analytics domains

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/config.py

@@ -38,6 +38,9 @@ class Settings(BaseSettings):
     rate_limit_burst: int = 10  # Allow burst of requests
     max_request_size: int = 1024 * 1024  # 1MB max request size
 
+    # Analytics
+    ga_measurement_id: Optional[str] = "G-J0PCVT14NY"  # Google Analytics 4 Measurement ID
+
     class Config:
         env_file = ".env"
         env_file_encoding = "utf-8"