Add robust input validation system and fix test suite

- Add comprehensive validation package with email (RFC 5322), password strength, and IANA timezone validation
- Implement strict password requirements: min 8 chars, uppercase, lowercase, digit, special character
- Integrate validation into RegisterUserHandler with complete test coverage (59 validation tests + 25 handler tests)
- Fix pre-existing test failures:
  - Remove tests for non-existent HabitEntry.DeletedAt and Delete() methods
  - Replace deprecated HabitTypeQuantity with HabitTypeValue
  - Add missing FindByHabitIDAndDateRange mock implementation
- Remove hardcoded localhost:8080 from Swagger config for self-hosted flexibility

internal/domain/entities/habit_entry_test.go

@@ -31,10 +31,6 @@ func TestNewHabitEntry(t *testing.T) {
 	if entry.CompletedAt.IsZero() {
 		t.Error("CompletedAt should not be zero")
 	}
-
-	if entry.DeletedAt != nil {
-		t.Error("DeletedAt should be nil for new entry")
-	}
 }
 
 func TestNewHabitEntry_BooleanHabit(t *testing.T) {
@@ -48,20 +44,3 @@ func TestNewHabitEntry_BooleanHabit(t *testing.T) {
 	}
 }
 
-func TestHabitEntry_SoftDelete(t *testing.T) {
-	entry := NewHabitEntry("habit-123", time.Now(), nil)
-
-	if entry.DeletedAt != nil {
-		t.Error("New entry should not be deleted")
-	}
-
-	entry.Delete()
-
-	if entry.DeletedAt == nil {
-		t.Error("Entry should be deleted after calling SoftDelete()")
-	}
-
-	if entry.DeletedAt.After(time.Now()) {
-		t.Error("DeletedAt should not be in the future")
-	}
-}