feat(db): switch to discourse/postgres image (auto-upgrade)

Move the db off the bitnami-era pgvector:pg17 + hand-rolled pg_upgrade entrypoint
to discourse/postgres:pg18 (pgvector + discourse's auto-upgrade layer). The image
runs the in-place major-version pg_upgrade itself on boot; the recipe configures it
via env:

- a small inline entrypoint injects the db password secret into $DB_PASSWORD (the
  image expects it in the env, no *_FILE support)
- POSTGRES_USER (the install user pg_upgrade must match) defaults to 'postgres' --
  correct for fresh installs and bitnami-origin clusters -- overridable from .env
- POSTGRES_INITDB_ARGS=--no-data-checksums so the new pg18 cluster matches pre-18
  clusters (pg18 initdb enables checksums by default; pg_upgrade needs a match)

- mount postgresql_data at /var/lib/postgresql (versioned PGDATA .../18/docker)
- pg_backup.sh uses POSTGRES_USER for the dump/drop/recreate; fix paths
- document the POSTGRES_USER override in .env.sample, README and the release note
- drop entrypoint.postgres.sh.tmpl

Tested on cctest: pg17->pg18 upgrade preserves data and serves over HTTPS; fresh
install works; backup+restore round-trips.

compose.yml

@@ -63,35 +63,51 @@ 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); everything is driven by the env below.
+    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
-        mode: 0555
       - source: pg_backup
         target: /pg_backup.sh
         mode: 0555
-    entrypoint: /docker-entrypoint.sh
+    entrypoint:
+      - /bin/bash
+      - -c
+      - |
+        if [ -f /run/secrets/db_password ]; then
+          DB_PASSWORD="$$(cat /run/secrets/db_password)"
+          export DB_PASSWORD POSTGRES_PASSWORD="$$DB_PASSWORD"
+        fi
+        exec run-postgres.sh postgres
     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
+      # pg_upgrade runs as this role and initdb's the new cluster with it; it must
+      # match the OLD cluster's bootstrap superuser (oid 10). The image default
+      # `postgres` matches fresh installs and bitnami-origin clusters. Override in
+      # the app .env (POSTGRES_USER=...) only for a cluster bootstrapped differently.
+      - POSTGRES_USER=${POSTGRES_USER:-postgres}
+      # pg18's initdb enables data checksums by default, but pg13-17 clusters here
+      # have them off and pg_upgrade requires a match -> initialise without them.
+      - POSTGRES_INITDB_ARGS=--no-data-checksums
     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
+      start_period: 15m
     deploy:
       labels:
         backupbot.backup: "true"
@@ -138,10 +154,6 @@ configs:
   app_migrate_uploads:
     name: ${STACK_NAME}_app_migrate_uploads_${APP_MIGRATE_UPLOADS_VERSION}
     file: migrate-uploads.sh
-  db_entrypoint:
-    name: ${STACK_NAME}_db_entrypoint_${DB_ENTRYPOINT_VERSION}
-    file: entrypoint.postgres.sh.tmpl
-    template_driver: golang
   pg_backup:
     name: ${STACK_NAME}_pg_backup_${PG_BACKUP_VERSION}
     file: pg_backup.sh