fix: pin postgres to version 17 to avoid data directory incompatibility

Postgres 18+ changed the default data directory structure, causing
startup failures when using an existing volume mounted at
/var/lib/postgresql/data.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 2 people

docker-compose.yml

@@ -2,7 +2,7 @@ version: "3"
 
 services:
   postgres:
-    image: postgres
+    image: postgres:17
     healthcheck:
       test: psql postgres --command "select 1" -U postgres
     volumes: