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/queries/get_user_habits_test.go

@@ -4,6 +4,7 @@ import (
 	"apocapoc-api/internal/domain/repositories"
 	"context"
 	"testing"
+	"time"
 
 	"apocapoc-api/internal/domain/entities"
 	"apocapoc-api/internal/domain/value_objects"
@@ -333,6 +334,18 @@ func (m *mockGetUserHabitsRepo) CountByUserIDFiltered(ctx context.Context, userI
 	return count, nil
 }
 
+func (m *mockGetUserHabitsRepo) GetChangesSince(ctx context.Context, userID string, since time.Time) (*repositories.HabitChanges, error) {
+	return &repositories.HabitChanges{
+		Created: []*entities.Habit{},
+		Updated: []*entities.Habit{},
+		Deleted: []string{},
+	}, nil
+}
+
+func (m *mockGetUserHabitsRepo) SoftDelete(ctx context.Context, id string) error {
+	return nil
+}
+
 func TestGetUserHabitsHandler_WithFilters(t *testing.T) {
 	habit1 := entities.NewHabit("user-123", "Exercise", value_objects.HabitTypeBoolean, value_objects.FrequencyDaily, false, false)
 	habit1.ID = "habit-1"