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/schema.sql

@@ -96,6 +96,8 @@ CREATE TABLE database_services (
     plan VARCHAR(50) NOT NULL,
     region VARCHAR(50) NOT NULL,
     connection_url TEXT,
+    backup_schedule VARCHAR(100),
+    next_backup_at TIMESTAMP WITH TIME ZONE,
     created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
     updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
 );