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.

pg_backup.sh

@@ -1,44 +1,59 @@
 #!/bin/bash
 
-# Postgres backup/restore hook for the discourse `db` service.
+# Postgres backup/restore hook for the discourse `db` service (discourse/postgres image).
 
 set -e
 
-BACKUP_FILE='/var/lib/postgresql/data/backup.sql'
-export PGPASSWORD=$(cat "${POSTGRES_PASSWORD_FILE:-/run/secrets/db_password}")
-DB_USER="${POSTGRES_USER:-discourse}"
+# discourse/postgres keeps the live cluster at a versioned PGDATA under the
+# /var/lib/postgresql mount. Write the dump at the volume root so backupbot's
+# `postgresql_data.path: backup.sql` label captures it.
+BACKUP_FILE='/var/lib/postgresql/backup.sql'
+DATADIR="${PGDATA:-/var/lib/postgresql/18/docker}"
 DB_NAME="${POSTGRES_DB:-discourse}"
 
+# The bootstrap superuser (install user, oid 10) differs between deployments
+# (`postgres` on bitnami-origin clusters, `discourse` on others). Detect it at
+# runtime over the local trust socket rather than hard-coding a name.
+detect_superuser() {
+  local u name
+  for u in discourse postgres; do
+    name="$(psql -U "$u" -d "$DB_NAME" -tAc 'select rolname from pg_roles where oid = 10' 2>/dev/null | tr -d '[:space:]')"
+    if [ -n "$name" ]; then echo "$name"; return 0; fi
+  done
+  echo postgres
+}
+SU="$(detect_superuser)"
+
 function backup {
-  pg_dump -U "$DB_USER" "$DB_NAME" | gzip > "$BACKUP_FILE"
+  pg_dump -U "$SU" "$DB_NAME" | gzip > "$BACKUP_FILE"
 }
 
 function restore {
-  cd /var/lib/postgresql/data/
+  cd "$DATADIR"
 
   # Block all non-local connections so the running discourse app + sidekiq cannot reconnect and
   # interfere with the drop/recreate/reimport. Restored on exit.
   restore_hba() {
     cat pg_hba.conf.bak > pg_hba.conf
     rm -f pg_hba.conf.bak
-    su postgres -c 'pg_ctl reload'
+    su postgres -c "pg_ctl -D '$DATADIR' reload"
   }
   cp pg_hba.conf pg_hba.conf.bak
   echo 'local all all trust' > pg_hba.conf
-  su postgres -c 'pg_ctl reload'
+  su postgres -c "pg_ctl -D '$DATADIR' reload"
   trap restore_hba EXIT INT TERM
 
   # terminate any lingering local sessions before recreate
   # see https://stackoverflow.com/questions/5108876/kill-a-postgresql-session-connection
-  psql -U "$DB_USER" -d postgres -c \
+  psql -U "$SU" -d postgres -c \
     "SELECT pg_terminate_backend(pid) FROM pg_stat_activity WHERE datname='${DB_NAME}' AND pid<>pg_backend_pid();"
 
   # drop database and then recreate it
-  psql -U "$DB_USER" -d postgres -c "DROP DATABASE ${DB_NAME} WITH (FORCE);"
-  createdb -U "$DB_USER" "$DB_NAME"
+  psql -U "$SU" -d postgres -c "DROP DATABASE ${DB_NAME} WITH (FORCE);"
+  createdb -U "$SU" "$DB_NAME"
 
-  # reimport data 
-  gunzip -c "$BACKUP_FILE" | psql -U "$DB_USER" -d "$DB_NAME" -1 -v ON_ERROR_STOP=1 -f -
+  # reimport data
+  gunzip -c "$BACKUP_FILE" | psql -U "$SU" -d "$DB_NAME" -1 -v ON_ERROR_STOP=1 -f -
 }
 
 $@