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/entities/habit.go

@@ -19,7 +19,9 @@ type Habit struct {
 	IsNegative    bool
 	TargetValue   *float64
 	CreatedAt     time.Time
+	UpdatedAt     time.Time
 	ArchivedAt    *time.Time
+	DeletedAt     *time.Time
 }
 
 func NewHabit(
@@ -30,6 +32,7 @@ func NewHabit(
 	carryOver bool,
 	isNegative bool,
 ) *Habit {
+	now := time.Now()
 	return &Habit{
 		UserID:     userID,
 		Name:       name,
@@ -37,15 +40,31 @@ func NewHabit(
 		Frequency:  frequency,
 		CarryOver:  carryOver,
 		IsNegative: isNegative,
-		CreatedAt:  time.Now(),
+		CreatedAt:  now,
+		UpdatedAt:  now,
 	}
 }
 
 func (h *Habit) Archive() {
 	now := time.Now()
 	h.ArchivedAt = &now
+	h.UpdatedAt = now
 }
 
 func (h *Habit) IsActive() bool {
 	return h.ArchivedAt == nil
 }
+
+func (h *Habit) Delete() {
+	now := time.Now()
+	h.DeletedAt = &now
+	h.UpdatedAt = now
+}
+
+func (h *Habit) IsDeleted() bool {
+	return h.DeletedAt != nil
+}
+
+func (h *Habit) Touch() {
+	h.UpdatedAt = time.Now()
+}