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/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,