refactor: update environment configuration and documentation

- Modified `.env.example` to reflect new API URL structure and added CDN configuration for external storage.
- Updated `.gitignore` to include deployment secrets and certificate files, ensuring sensitive information is not committed.
- Revised `BACKEND_INTEGRATION.md` to clarify authentication methods, replacing Cognito references with Nostr NIP-98.
- Deleted outdated documentation files (`CONTENT-INTEGRATION-COMPLETE.md`, `CURSOR-MCP-SETUP.md`, `FINAL-STATUS.md`, `FIXES-APPLIED.md`, `INDEEHHUB-INTEGRATION.md`, `PROJECT-COMPLETE.md`, `PROJECT-SUMMARY.md`) to streamline project documentation.

These changes enhance the clarity of the environment setup and improve the overall documentation structure for better developer onboarding.

backend/package.json

@@ -111,6 +111,7 @@
     "class-validator": "^0.14.1",
     "dayjs": "^1.11.13",
     "fast-xml-parser": "^5.2.0",
+    "helmet": "^8.1.0",
     "jsonwebtoken": "^9.0.3",
     "jwks-rsa": "^3.0.1",
     "moment": "^2.30.1",