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/migrations_goose/20260919000000_database_backup_schedule.sql

@@ -0,0 +1,9 @@
+-- +goose Up
+ALTER TABLE public.database_services
+    ADD COLUMN IF NOT EXISTS backup_schedule character varying(100),
+    ADD COLUMN IF NOT EXISTS next_backup_at timestamp with time zone;
+
+-- +goose Down
+ALTER TABLE public.database_services
+    DROP COLUMN IF EXISTS backup_schedule,
+    DROP COLUMN IF EXISTS next_backup_at;