Add TLS/HTTPS support with auto-detection

- Server auto-detects cert/key in backend/certs/ and starts HTTPS
- Falls back to plain HTTP if no certs found or TLS_ENABLED=false
- Self-signed cert generated for dev (365-day, gitignored)
- Added TLS env vars to .env.example
- Frontend rebuilt with https:// API URLs for dev server

backend/.gitignore

@@ -3,3 +3,6 @@
 backend/fix_multivendor_constraint.js
 backend/migrate_multivendor.js
 backend/add_vendor_to_documents.js
+
+# TLS certificates (self-signed or CA-issued)
+certs/