feat: implement offline sync endpoints with Last-Write-Wins strategy

Add comprehensive offline synchronization support for habits and entries:

## Infrastructure (Phase 1)
- Add UpdatedAt and DeletedAt timestamps to Habit and HabitEntry entities
- Implement soft delete with Delete(), Touch(), and IsDeleted() methods
- Create SQL migration with optimized composite indexes for sync queries
- Add GetChangesSince() and SoftDelete() to both repositories
- Update all Find* methods to exclude soft-deleted records
- 13 comprehensive TDD tests for sync repository methods

## HTTP Endpoints (Phase 2)
- GET /api/v1/sync/changes: retrieve all changes since timestamp
- POST /api/v1/sync/batch: apply client changes with conflict resolution
- Implement Last-Write-Wins strategy using UpdatedAt timestamps
- Add authentication and rate limiting (100 req/min)
- Validate user ownership for all sync operations
- 9 tests for sync handlers (3 queries + 6 commands)

## Technical Details
- Composite indexes: (user_id, updated_at) for optimal query performance
- No pagination: atomic sync operations for data consistency
- Upsert behavior: create resources if not found on server
- DTOs with full entity state including timestamps
- Swagger documentation updated for new endpoints

All 220+ tests passing ✓

internal/application/commands/unmark_habit_test.go

@@ -155,3 +155,15 @@ func (m *mockEntryRepoForUnmark) FindByUserIDFiltered(ctx context.Context, userI
 func (m *mockEntryRepoForUnmark) CountByUserIDFiltered(ctx context.Context, userID string, filter repositories.HabitFilter) (int, error) {
 	return 0, nil
 }
+
+func (m *mockEntryRepoForUnmark) GetChangesSince(ctx context.Context, userID string, since time.Time) (*repositories.HabitEntryChanges, error) {
+	return &repositories.HabitEntryChanges{
+		Created: []*entities.HabitEntry{},
+		Updated: []*entities.HabitEntry{},
+		Deleted: []string{},
+	}, nil
+}
+
+func (m *mockEntryRepoForUnmark) SoftDelete(ctx context.Context, id string) error {
+	return nil
+}