Implement SQLite persistence layer (TDD)

- Add SQLite and UUID dependencies (go-sqlite3, google/uuid)
- Create database connection with automatic migrations
- Implement UserRepository with full CRUD operations
- Implement HabitRepository with JSON serialization for arrays
- Implement HabitEntryRepository with date range queries
- Add comprehensive test coverage for all repositories
- Fix User entity to default timezone to UTC when empty
- All tests passing with TDD approach (Red-Green-Refactor)

internal/domain/entities/habit_entry_test.go

@@ -55,7 +55,7 @@ func TestHabitEntry_SoftDelete(t *testing.T) {
 		t.Error("New entry should not be deleted")
 	}
 
-	entry.SoftDelete()
+	entry.Delete()
 
 	if entry.DeletedAt == nil {
 		t.Error("Entry should be deleted after calling SoftDelete()")

internal/domain/entities/user.go

@@ -13,6 +13,9 @@ type User struct {
 
 func NewUser(email, passwordHash, timezone string) *User {
 	now := time.Now()
+	if timezone == "" {
+		timezone = "UTC"
+	}
 	return &User{
 		Email:        email,
 		PasswordHash: passwordHash,