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- Deleted outdated documentation files (`CONTENT-INTEGRATION-COMPLETE.md`, `CURSOR-MCP-SETUP.md`, `FINAL-STATUS.md`, `FIXES-APPLIED.md`, `INDEEHHUB-INTEGRATION.md`,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-lock.json

@@ -49,6 +49,7 @@
         "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",
@@ -14299,6 +14300,15 @@
         "node": ">= 0.4"
       }
     },
+    "node_modules/helmet": {
+      "version": "8.1.0",
+      "resolved": "https://registry.npmjs.org/helmet/-/helmet-8.1.0.tgz",
+      "integrity": "sha512-jOiHyAZsmnr8LqoPGmCjYAaiuWwjAPLgY8ZX2XrmHawt99/u1y6RgrZMTeoPfpUbV96HOalYgz1qzkRbw54Pmg==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=18.0.0"
+      }
+    },
     "node_modules/homedir-polyfill": {
       "version": "1.0.3",
       "resolved": "https://registry.npmjs.org/homedir-polyfill/-/homedir-polyfill-1.0.3.tgz",