fix(db): switch postgres image to pgvector/pgvector:pg17 + bump PG_BACKUP_VERSION

- Switch db image from postgres:16 to pgvector/pgvector:pg17. The
  bundled discourse-ai plugin's migration 20230710171141 runs
  'CREATE EXTENSION IF NOT EXISTS vector' on every boot; without
  the extension binaries installed in the postgres image, the
  migration transaction is aborted and the app crash-loops. The
  pgvector image is debian-based (bookworm) with the postgres
  apt repo + pgdg.list already configured, so the recipe's
  homegrown pg_upgrade entrypoint continues to work across major
  version bumps.
- Bump PG_BACKUP_VERSION v1 → v2: the prior 'improved comments'
  commit edited pg_backup.sh content without bumping the config
  version, so any operator redeploying without a fresh stack
  would hit 'only updates to Labels are allowed' from docker
  swarm (configs are immutable; content change needs a new name).

abra.sh

@@ -1,2 +1,2 @@
 export DB_ENTRYPOINT_VERSION=v1
-export PG_BACKUP_VERSION=v1
+export PG_BACKUP_VERSION=v2

compose.yml

@@ -52,7 +52,7 @@ services:
       start_period: 20m
 
   db:
-    image: postgres:16
+    image: pgvector/pgvector:pg17
     networks:
       - internal
     secrets: