Implement Issue #2: Scheduling & Publishing

Replaces stub schedule CRUD with full shift template + instance system.

- DB: add shift_templates, shift_template_roles, shift_template_volunteers,
  shift_instances, shift_instance_volunteers tables
- schedule package: ShiftTemplate and ShiftInstance models with store
  (generate, publish/unpublish, per-instance edits, volunteer confirmation)
- API: shift-templates CRUD + shifts generate/publish/unpublish/update/confirm
- Notifications sent on publish (FR-S04), unpublish (FR-S05), instance edit
  (FR-S09), and volunteer added mid-month (FR-S10)
- Frontend: Schedules page with month navigation, template management,
  publish/unpublish controls, and per-shift edit/confirm
- Tests: Go handler tests (14 cases) + React tests (11 cases)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/notification/notification.go

@@ -77,6 +77,12 @@ func (s *Store) ListForVolunteer(ctx context.Context, volunteerID int64) ([]Noti
 	return notifications, rows.Err()
 }
 
+// CreateNotification satisfies the schedule.Notifier interface.
+func (s *Store) CreateNotification(ctx context.Context, volunteerID int64, message string) error {
+	_, err := s.Create(ctx, volunteerID, message)
+	return err
+}
+
 func (s *Store) MarkRead(ctx context.Context, id, volunteerID int64) (*Notification, error) {
 	result, err := s.db.ExecContext(ctx,
 		`UPDATE notifications SET read = 1 WHERE id = ? AND volunteer_id = ?`,