chore(env): unify to MONGODB_URI with MONGO_URI fallback; merge env examples

.env.example

@@ -1,6 +1,25 @@
-# Copy to .env and fill values
-MONGO_URI=mongodb://mongo:27017/chatdb
+###############################
+# Environment Example (copy to .env)
+###############################
+# Unified Mongo variable name:
+#   MONGODB_URI is the standard now.
+# Legacy fallback MONGO_URI is still read by code but will be removed later.
+MONGODB_URI=mongodb://mongo:27017/chatdb
+# (Optional legacy) MONGO_URI=mongodb://mongo:27017/chatdb
+
+# Security: change this to a long random string (at least 32 chars)
 JWT_SECRET=change_this_to_a_random_secret
+
+# Server port (container internal port already mapped in docker-compose)
 PORT=4000
+
+# OpenAI / Compatible API
 OPENAI_API_KEY=
+OPENAI_BASE_URL=https://api.openai.com
+
+# DeepSeek (OpenAI-compatible)
+DEEPSEEK_API_KEY=
+DEEPSEEK_BASE_URL=https://api.deepseek.com
+
+# Other provider placeholder
 OTHER_PROVIDER_KEY=