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/domain/repositories/habit_repository.go

@@ -2,6 +2,7 @@ package repositories
 
 import (
 	"context"
+	"time"
 
 	"apocapoc-api/internal/domain/entities"
 	"apocapoc-api/internal/domain/value_objects"
@@ -15,6 +16,12 @@ type HabitFilter struct {
 	Search          string
 }
 
+type HabitChanges struct {
+	Created []*entities.Habit
+	Updated []*entities.Habit
+	Deleted []string
+}
+
 type HabitRepository interface {
 	Create(ctx context.Context, habit *entities.Habit) error
 	FindByID(ctx context.Context, id string) (*entities.Habit, error)
@@ -26,4 +33,8 @@ type HabitRepository interface {
 	CountByUserIDFiltered(ctx context.Context, userID string, filter HabitFilter) (int, error)
 	Update(ctx context.Context, habit *entities.Habit) error
 	Delete(ctx context.Context, id string) error
+
+	// Sync methods
+	GetChangesSince(ctx context.Context, userID string, since time.Time) (*HabitChanges, error)
+	SoftDelete(ctx context.Context, id string) error
 }