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>

.env.example

@@ -163,3 +163,38 @@ FLOWER_URL=http://localhost:5555
 # Flower basic authentication password
 # ⚠️  CHANGE THIS IN PRODUCTION!
 FLOWER_PASSWORD=changeme
+
+# =============================================================================
+# SENTRY ERROR TRACKING
+# =============================================================================
+# Get your DSN from https://sentry.io (free tier available)
+# Leave empty to disable Sentry
+SENTRY_DSN=
+SENTRY_ENVIRONMENT=production
+SENTRY_TRACES_SAMPLE_RATE=0.1
+
+# =============================================================================
+# CLOUDFLARE R2 STORAGE
+# =============================================================================
+# Storage backend: "local" (default) or "r2" for Cloudflare R2
+# Set to "r2" for production to enable cloud storage
+STORAGE_BACKEND=local
+
+# Cloudflare R2 credentials (required when STORAGE_BACKEND=r2)
+# Get these from Cloudflare Dashboard > R2 > Manage R2 API Tokens
+R2_ACCOUNT_ID=
+R2_ACCESS_KEY_ID=
+R2_SECRET_ACCESS_KEY=
+R2_BUCKET_NAME=wizamart-media
+
+# Public URL for R2 bucket (optional - for custom domain)
+# If not set, uses Cloudflare's default R2 public URL
+# Example: https://media.yoursite.com
+R2_PUBLIC_URL=
+
+# =============================================================================
+# CLOUDFLARE CDN / PROXY
+# =============================================================================
+# Set to true when your domain is proxied through CloudFlare
+# This enables proper handling of CF-Connecting-IP and other CloudFlare headers
+CLOUDFLARE_ENABLED=false