refactor: extract backup/restore into config scripts, trim comments

Move the postgres and clickhouse backup/restore hook logic out of inline
compose labels into dedicated pg_backup.sh / clickhouse_backup.sh config
scripts (the pattern other recipes use), and trim the verbose explanatory
comments down to the essential rationale, now living in the scripts.

clickhouse_backup.sh

@@ -0,0 +1,30 @@
+#!/bin/bash
+
+set -e
+
+# clickhouse-backup output lives inside the event-data volume (snapshotted via
+# backupbot.backup.volumes.event-data.path). Restoring the raw data files under a
+# running server is unsafe, so restore performs a logical restore instead.
+BACKUP_DIR=/var/lib/clickhouse/backup/events
+MIGRATIONS_TSV="$BACKUP_DIR/schema_migrations.tsv"
+
+backup() {
+  clickhouse-backup create events
+  # schema_migrations is a TinyLog table — clickhouse-backup only FREEZEs MergeTree
+  # data, so its rows aren't captured. Export them alongside the backup, else a restore
+  # leaves the ledger empty and the next boot re-runs every migration (DUPLICATE_COLUMN).
+  clickhouse-client --query "SELECT * FROM plausible_events_db.schema_migrations FORMAT TSV" > "$MIGRATIONS_TSV"
+}
+
+backup_cleanup() {
+  rm -rf "$BACKUP_DIR"
+}
+
+restore() {
+  clickhouse-backup restore --rm events
+  clickhouse-client --query "TRUNCATE TABLE plausible_events_db.schema_migrations"
+  clickhouse-client --query "INSERT INTO plausible_events_db.schema_migrations FORMAT TSV" < "$MIGRATIONS_TSV"
+  rm -rf "$BACKUP_DIR"
+}
+
+"$@"