feat(backups): scheduled database backups via cron tick

- database_services gains backup_schedule + next_backup_at (goose migration,
  sqlc schema, new queries).
- PUT /databases/:id accepts backup_schedule — validated cron expression,
  empty string disables; invalid returns 400.
- Cron scheduler tick now also folds due databases into the existing
  snapshot pipeline (createBackupProcess).
- Databases page backups section gets a schedule row showing current expr
  and next run.
- resolveBackupConfig reports real next_backup/enabled instead of a fake
  last+24h estimate.
- Verified e2e: '* * * * *' produced three consecutive completed archives
  containing the seeded payload; invalid schedule rejected.

Authored By: TDvorak <[email protected]>

app/backend/sqlc/queries/databases.sql

@@ -1,11 +1,11 @@
 -- name: ListDatabaseServicesByUser :many
-SELECT id, name, type, status, version, plan, region, connection_url, created_at, updated_at
+SELECT id, name, type, status, version, plan, region, connection_url, backup_schedule, next_backup_at, created_at, updated_at
 FROM database_services
 WHERE user_id = $1
 ORDER BY created_at DESC;
 
 -- name: GetDatabaseServiceByIDAndUser :one
-SELECT id, name, type, status, version, plan, region, connection_url, created_at, updated_at
+SELECT id, name, type, status, version, plan, region, connection_url, backup_schedule, next_backup_at, created_at, updated_at
 FROM database_services
 WHERE id = $1 AND user_id = $2;
 
@@ -79,3 +79,19 @@ VALUES ($1, $2, $3, $4, $5, $6);
 UPDATE database_backups
 SET status = $1, size = $2, completed_at = $3
 WHERE id = $4;
+
+-- name: SetDatabaseBackupScheduleByIDAndUser :exec
+UPDATE database_services
+SET backup_schedule = $1, next_backup_at = $2, updated_at = $3
+WHERE id = $4 AND user_id = $5;
+
+-- name: ListDueDatabaseBackups :many
+SELECT id, backup_schedule
+FROM database_services
+WHERE backup_schedule IS NOT NULL AND backup_schedule <> ''
+  AND next_backup_at IS NOT NULL AND next_backup_at <= NOW();
+
+-- name: SetDatabaseNextBackupAt :exec
+UPDATE database_services
+SET next_backup_at = $1
+WHERE id = $2;