feat(db): use pgautoupgrade image instead of pgvector + bespoke pg_upgrade

discourse core does not use the vector extension, so the pgvector image is
unnecessary. Switch db to pgautoupgrade/pgautoupgrade:17-trixie, which handles
major-version upgrades (old binaries, initdb, pg_upgrade) itself.

pgautoupgrade assumes the old cluster's install user (oid 10) equals
POSTGRES_USER and fails pg_upgrade otherwise (issue #115). A thin wrapper
entrypoint detects the old install user when an upgrade is pending and exports
POSTGRES_USER so pgautoupgrade matches it. Replaces the 64-line hand-rolled
pg_upgrade entrypoint.

compose.yml

@@ -63,7 +63,7 @@ services:
       start_period: 25m
 
   db:
-    image: pgvector/pgvector:pg17
+    image: pgautoupgrade/pgautoupgrade:17-trixie
     networks:
       - internal
     secrets:
@@ -72,25 +72,26 @@ services:
       - 'postgresql_data:/var/lib/postgresql/data'
     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:
       - POSTGRES_HOST_AUTH_METHOD=trust
       - POSTGRES_USER=discourse
       - POSTGRES_DB=discourse
       - POSTGRES_PASSWORD_FILE=/run/secrets/db_password
+      - PGTARGET=17
     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
+      # generous: pgautoupgrade may run an in-place pg_upgrade on deploy before
+      # the server accepts connections — don't let the healthcheck kill an
+      # in-progress migration
       start_period: 10m
     deploy:
       labels:
@@ -140,8 +141,7 @@ 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