Fix staging auto-deploy: env write + port isolation (#3)

docker-compose.staging.yml

@@ -21,7 +21,9 @@
 
 services:
   postgres:
-    ports:
+    # !override replaces the base port list instead of merging with it, so staging
+    # binds ONLY its shifted 18xxx/15432 ports and never squats on prod's 8080/8081.
+    ports: !override
       - "127.0.0.1:15432:5432"
 
   api:
@@ -32,13 +34,13 @@ services:
       App__DevMode: "true"
       GmailSync__MaxMessages: ${MAX_MESSAGES:-2000}
       Cors__Origins__0: ${FRONTEND_ORIGIN:-http://localhost:18081}
-    ports:
+    ports: !override
       - "127.0.0.1:18080:8080"
 
   frontend:
-    ports:
+    ports: !override
       - "18081:80"
 
   nginx:
-    ports:
+    ports: !override
       - "18000:80"