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- Deleted outdated documentation files (`CONTENT-INTEGRATION-COMPLETE.md`, `CURSOR-MCP-SETUP.md`, `FINAL-STATUS.md`, `FIXES-APPLIED.md`, `INDEEHHUB-INTEGRATION.md`,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

.gitignore

@@ -28,5 +28,15 @@ dist-ssr
 .env.local
 .env.*.local
 
+# Portainer / deployment secrets (NEVER commit)
+.env.portainer
+.env.portainer.*
+!.env.portainer.example
+
+# Keys and certs
+*.pem
+*.key
+*.p12
+
 # Build
 build