feat: add Sentry, Cloudflare R2, and CloudFlare CDN integrations

Production quick wins for improved observability and scalability:

Sentry Error Tracking:
- Add sentry-sdk[fastapi] dependency
- Initialize Sentry in main.py with FastAPI/SQLAlchemy integrations
- Add Celery integration for background task error tracking
- Feature-flagged via SENTRY_DSN (disabled when empty)

Cloudflare R2 Storage:
- Add boto3 dependency for S3-compatible API
- Create storage_service.py with StorageBackend abstraction
- LocalStorageBackend for development (default)
- R2StorageBackend for production cloud storage
- Feature-flagged via STORAGE_BACKEND setting

CloudFlare CDN/Proxy:
- Create middleware/cloudflare.py for CF header handling
- Extract real client IP from CF-Connecting-IP
- Support CF-IPCountry for geo features
- Feature-flagged via CLOUDFLARE_ENABLED setting

Documentation:
- Add docs/deployment/cloudflare.md setup guide
- Update infrastructure.md with dev vs prod requirements
- Add enterprise upgrade checklist for scaling beyond 1000 users
- Update installation.md with new environment variables

All features are optional and disabled by default for development.

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

docs/index.md

@@ -435,9 +435,12 @@ We welcome contributions! Please see our [Contributing Guide](development/contri
 - ✅ Marketplace integration
 - ✅ Team management
 
+**Production Ready**:
+- ✅ Error tracking (Sentry integration)
+- ✅ Cloud storage (Cloudflare R2)
+- ✅ CDN & WAF (CloudFlare)
+
 **In Development**:
-- 🚧 Payment integration (Stripe-ready)
-- 🚧 Email notifications
 - 🚧 Advanced analytics
 
 ---