fix(backup): back up the postgres database (was unprotected)

The immich recipe declared backupbot.backup only on the app service (whose volumes
are all excluded), so the postgres database — all of immich users metadata, albums,
people, settings — was NOT backed up at all. Restoring a backup yielded an empty DB.

Add a database-service postgres backup mirroring the coop-cloud convention
(matrix-synapse, keycloak, ...): a /pg_backup.sh config-mounted hook driven by
backupbot pre/restore hooks. backup = pg_dump | gzip -> backup.sql in the postgres
volume; restore = terminate immich-server connections, FORCE-drop, recreate, reimport
(the VectorChord/pgvecto.rs extensions and all data round-trip cleanly).

abra.sh

@@ -0,0 +1 @@
+export PG_BACKUP_VERSION=v1

compose.yml

@@ -67,6 +67,21 @@ services:
       - postgres:/var/lib/postgresql/data
     networks:
       - backend
+    deploy:
+      labels:
+        backupbot.backup: "${ENABLE_BACKUPS:-true}"
+        backupbot.backup.pre-hook: "/pg_backup.sh backup"
+        backupbot.backup.volumes.postgres.path: "backup.sql"
+        backupbot.restore.post-hook: "/pg_backup.sh restore"
+    configs:
+      - source: pg_backup
+        target: /pg_backup.sh
+        mode: 0555
+
+configs:
+  pg_backup:
+    name: ${STACK_NAME}_pg_backup_${PG_BACKUP_VERSION}
+    file: pg_backup.sh
 
 secrets:
   db_password:

pg_backup.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+
+# Postgres backup/restore hook for the immich `database` service (VectorChord/pgvecto.rs image).
+# Invoked by backupbot-two via the deploy labels:
+#   backupbot.backup.pre-hook    = "/pg_backup.sh backup"
+#   backupbot.backup.volumes.postgres.path = "backup.sql"
+#   backupbot.restore.post-hook  = "/pg_backup.sh restore"
+# Backup dumps the immich DB to backup.sql (gzip) inside the postgres volume; backupbot then
+# archives that file. Restore reads it back and reimports. immich-server keeps TCP connections
+# open to the DB, so restore must terminate them and FORCE-drop before recreating (the matrix-synapse
+# pg_hba "local trust" trick does not cover networked connections).
+
+set -e
+
+BACKUP_FILE='/var/lib/postgresql/data/backup.sql'
+export PGPASSWORD=$(cat "${POSTGRES_PASSWORD_FILE:-/run/secrets/db_password}")
+DB_USER="${POSTGRES_USER:-postgres}"
+DB_NAME="${POSTGRES_DB:-immich}"
+
+function backup {
+  pg_dump -U "$DB_USER" "$DB_NAME" | gzip > "$BACKUP_FILE"
+}
+
+function restore {
+  # immich-server holds connections to the DB; drop them so DROP DATABASE can proceed.
+  psql -U "$DB_USER" -d postgres -c \
+    "SELECT pg_terminate_backend(pid) FROM pg_stat_activity WHERE datname='${DB_NAME}' AND pid<>pg_backend_pid();"
+  psql -U "$DB_USER" -d postgres -c "DROP DATABASE ${DB_NAME} WITH (FORCE);"
+  createdb -U "$DB_USER" "$DB_NAME"
+  gunzip -c "$BACKUP_FILE" | psql -U "$DB_USER" -d "$DB_NAME" -1 -v ON_ERROR_STOP=1 -f -
+}
+
+$@