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/internal/database/sqlcdb/models.go

@@ -80,17 +80,19 @@ type DatabaseBackup struct {
 }
 
 type DatabaseService struct {
-	ID            string         `json:"id"`
-	UserID        string         `json:"user_id"`
-	Name          string         `json:"name"`
-	Type          string         `json:"type"`
-	Status        string         `json:"status"`
-	Version       string         `json:"version"`
-	Plan          string         `json:"plan"`
-	Region        string         `json:"region"`
-	ConnectionUrl sql.NullString `json:"connection_url"`
-	CreatedAt     sql.NullTime   `json:"created_at"`
-	UpdatedAt     sql.NullTime   `json:"updated_at"`
+	ID             string         `json:"id"`
+	UserID         string         `json:"user_id"`
+	Name           string         `json:"name"`
+	Type           string         `json:"type"`
+	Status         string         `json:"status"`
+	Version        string         `json:"version"`
+	Plan           string         `json:"plan"`
+	Region         string         `json:"region"`
+	ConnectionUrl  sql.NullString `json:"connection_url"`
+	BackupSchedule sql.NullString `json:"backup_schedule"`
+	NextBackupAt   sql.NullTime   `json:"next_backup_at"`
+	CreatedAt      sql.NullTime   `json:"created_at"`
+	UpdatedAt      sql.NullTime   `json:"updated_at"`
 }
 
 type Deployment struct {