chore: switch prod PostgreSQL to pgvector image and secure .env.production

.gitignore

@@ -16,8 +16,9 @@ backend/.venv/
 .env
 backend/.env
 frontend/.env.local
-# 不忽略 .env.example（供新成员参考）
+.env.production
 !.env.example
+!.env.production.example
 
 # IDE
 .idea/

docker-compose.prod.yml

@@ -17,7 +17,7 @@ version: "3.9"
 
 services:
   db:
-    image: postgres:15-alpine
+    image: pgvector/pgvector:pg15
     container_name: geo_db_prod
     restart: always
     environment: