feat(db): switch to discourse/postgres image with install-user + checksum adapter

Replace the bitnami-era pgvector:pg17 db + hand-rolled pg_upgrade entrypoint
with discourse/postgres:pg18 (pgvector + discourse's auto-upgrade layer, as
suggested on coop-cloud/discourse#16). The image does the heavy lifting
(installs old binaries, runs pg_upgrade into the versioned PGDATA); a thin
cc-db-entrypoint.sh wrapper fills the two gaps it leaves:

- secrets: inject DB_PASSWORD/POSTGRES_PASSWORD from the docker secret (the
  image reads them from env, no *_FILE support);
- install user: detect the old cluster's bootstrap superuser (oid 10) and
  export POSTGRES_USER so pg_upgrade + the new cluster's initdb match it. Real
  deployments differ (bitnami-origin clusters install as 'postgres' + a
  'discourse' app role; others as 'discourse'). The image hardcodes
  --username=$POSTGRES_USER and never detects this, so the adapter is required;
- checksums: pg18's initdb enables data checksums by default but pg13-17
  clusters here have them off, and pg_upgrade requires a match -> initdb the new
  cluster with --no-data-checksums unless the old one reports them on.

Other changes:
- mount postgresql_data at /var/lib/postgresql (versioned PGDATA .../18/docker)
- pg_backup.sh: detect the superuser at runtime; fix paths for the new layout
- bump DB_ENTRYPOINT_VERSION v6, PG_BACKUP_VERSION v3 (immutable swarm configs)
- drop entrypoint.postgres.sh.tmpl

Verified on cctest: upgrade from an existing pg17 cluster (install user
'postgres') -> pg18, all data preserved, serves over HTTPS via Traefik.

compose.yml

@@ -63,35 +63,42 @@ services:
       start_period: 25m
 
   db:
-    image: pgvector/pgvector:pg17
+    # discourse/postgres = pgvector + discourse's postgres management layer, which
+    # auto-upgrades an older cluster in place on boot (pg_upgrade into the versioned
+    # PGDATA /var/lib/postgresql/${MAJOR}/docker). The cc-db-entrypoint wrapper
+    # injects the password secret and detects the old cluster's install user.
+    image: discourse/postgres:pg18
     networks:
       - internal
     secrets:
       - db_password
     volumes:
-      - 'postgresql_data:/var/lib/postgresql/data'
+      # the image expects the whole cluster tree mounted here (not the data subdir);
+      # an existing pg17 cluster at the volume root is found and upgraded into /18/docker
+      - 'postgresql_data:/var/lib/postgresql'
     configs:
       - source: db_entrypoint
-        target: /docker-entrypoint.sh
+        target: /usr/local/bin/cc-db-entrypoint.sh
         mode: 0555
       - source: pg_backup
         target: /pg_backup.sh
         mode: 0555
-    entrypoint: /docker-entrypoint.sh
+    entrypoint: /usr/local/bin/cc-db-entrypoint.sh
     environment:
+      # internal-only overlay network; keep all-trust so the app and the
+      # backup/restore hooks connect without juggling the superuser password
       - POSTGRES_HOST_AUTH_METHOD=trust
-      - POSTGRES_USER=discourse
       - POSTGRES_DB=discourse
-      - POSTGRES_PASSWORD_FILE=/run/secrets/db_password
+      - DB_USER=discourse
     healthcheck:
       test: "pg_isready -U discourse -d discourse"
       interval: 30s
       timeout: 10s
       retries: 5
-      # generous: a postgres major-version upgrade (apt install + pg_upgrade) runs
-      # in the entrypoint before the server accepts connections — don't let the
-      # healthcheck kill an in-progress migration
-      start_period: 10m
+      # generous: a postgres major-version upgrade (apt install old binaries +
+      # pg_upgrade) runs in the entrypoint before the server accepts connections —
+      # don't let the healthcheck kill an in-progress migration
+      start_period: 15m
     deploy:
       labels:
         backupbot.backup: "true"
@@ -140,8 +147,7 @@ configs:
     file: migrate-uploads.sh
   db_entrypoint:
     name: ${STACK_NAME}_db_entrypoint_${DB_ENTRYPOINT_VERSION}
-    file: entrypoint.postgres.sh.tmpl
-    template_driver: golang
+    file: cc-db-entrypoint.sh
   pg_backup:
     name: ${STACK_NAME}_pg_backup_${PG_BACKUP_VERSION}
     file: pg_backup.sh