Add data export functionality with gzip compression

- Add FindByUserID method to HabitEntryRepository (JOIN with habits)
- Implement ExportUserDataHandler to export all user data
- Add GET /api/v1/export endpoint with gzip compression
- Apply strict rate limiting (1 export per hour per user)
- Export includes all habits (active + archived) and entries
- Add i18n translations for export errors (en/es)
- Update all test mocks to implement new repository method
- Export format: JSON with gzip (~10x compression ratio)

internal/application/commands/mark_habit_test.go

@@ -40,6 +40,10 @@ func (m *mockEntryRepo) FindByHabitIDAndDateRange(ctx context.Context, habitID s
 	return nil, nil
 }
 
+func (m *mockEntryRepo) FindByUserID(ctx context.Context, userID string) ([]*entities.HabitEntry, error) {
+	return nil, nil
+}
+
 func (m *mockEntryRepo) FindPendingByHabitID(ctx context.Context, habitID string, beforeDate time.Time) ([]*entities.HabitEntry, error) {
 	return nil, nil
 }