Fix polymorphic tagging and add test script

This commit fixes the tag loading issue and adds comprehensive testing.

## Bug Fix: Polymorphic Tag Loading

Fixed issue with many-to-many tag relationships not working correctly with
polymorphic associations. The problem was that GORM doesn't support using
`many2many` with polymorphic relationships directly.

**Changes:**
- Modified `internal/models/todo.go`: Changed Tags field to use `gorm:"-"`
  to skip GORM handling
- Modified `internal/models/recurring_todo.go`: Same fix for recurring todos
- Modified `internal/services/todo_service.go`: Added `loadTodoTags()` helper
  function to manually load tags through the taggings join table

**How it works now:**
1. Tags are no longer automatically loaded by GORM
2. Manual loading via JOIN query: `tags JOIN taggings ON tag_id WHERE taggable_id AND taggable_type`
3. Called after loading todos in both `GetTodo()` and `GetTodos()`

## Testing

Added `test_api.sh` - comprehensive integration test script that tests:
1. Health check
2. User registration
3. Authentication
4. Context creation
5. Project creation
6. Todo creation with tags
7. Listing todos with filters
8. Completing todos
9. Project statistics

All tests pass successfully!

## Files Changed

- `internal/models/todo.go`: Fix tag relationship
- `internal/models/recurring_todo.go`: Fix tag relationship
- `internal/services/todo_service.go`: Add manual tag loading
- `test_api.sh`: New integration test script
- `go.sum`: Updated with exact dependency versions

internal/models/recurring_todo.go

@@ -72,7 +72,7 @@ type RecurringTodo struct {
 	Project  *Project  `gorm:"foreignKey:ProjectID" json:"project,omitempty"`
 	Todos    []Todo    `gorm:"foreignKey:RecurringTodoID" json:"todos,omitempty"`
 	Taggings []Tagging `gorm:"polymorphic:Taggable" json:"-"`
-	Tags     []Tag     `gorm:"many2many:taggings;foreignKey:ID;joinForeignKey:TaggableID;References:ID;joinReferences:TagID" json:"tags,omitempty"`
+	Tags     []Tag     `gorm:"-" json:"tags,omitempty"`
 }
 
 // BeforeCreate sets default values

internal/models/todo.go

@@ -41,7 +41,7 @@ type Todo struct {
 	Project       *Project      `gorm:"foreignKey:ProjectID" json:"project,omitempty"`
 	RecurringTodo *RecurringTodo `gorm:"foreignKey:RecurringTodoID" json:"recurring_todo,omitempty"`
 	Taggings      []Tagging     `gorm:"polymorphic:Taggable" json:"-"`
-	Tags          []Tag         `gorm:"many2many:taggings;foreignKey:ID;joinForeignKey:TaggableID;References:ID;joinReferences:TagID" json:"tags,omitempty"`
+	Tags          []Tag         `gorm:"-" json:"tags,omitempty"`
 	Attachments   []Attachment  `gorm:"foreignKey:TodoID" json:"attachments,omitempty"`
 
 	// Dependencies

internal/services/todo_service.go

@@ -96,10 +96,6 @@ func (s *TodoService) GetTodos(userID uint, filter ListTodosFilter) ([]models.To
 	// Preload associations
 	query = query.Preload("Context").Preload("Project")
 
-	if filter.IncludeTags {
-		query = query.Preload("Tags")
-	}
-
 	// Filter by tag
 	if filter.TagName != nil {
 		query = query.Joins("JOIN taggings ON taggings.taggable_id = todos.id AND taggings.taggable_type = ?", "Todo").
@@ -114,6 +110,15 @@ func (s *TodoService) GetTodos(userID uint, filter ListTodosFilter) ([]models.To
 		return nil, err
 	}
 
+	// Load tags if requested
+	if filter.IncludeTags {
+		for i := range todos {
+			if err := s.loadTodoTags(&todos[i]); err != nil {
+				return nil, err
+			}
+		}
+	}
+
 	return todos, nil
 }
 
@@ -125,7 +130,6 @@ func (s *TodoService) GetTodo(userID, todoID uint) (*models.Todo, error) {
 		Where("id = ? AND user_id = ?", todoID, userID).
 		Preload("Context").
 		Preload("Project").
-		Preload("Tags").
 		Preload("Attachments").
 		Preload("Predecessors.Predecessor").
 		Preload("Successors.Successor").
@@ -136,9 +140,31 @@ func (s *TodoService) GetTodo(userID, todoID uint) (*models.Todo, error) {
 		return nil, err
 	}
 
+	// Load tags manually through taggings
+	if err := s.loadTodoTags(&todo); err != nil {
+		return nil, err
+	}
+
 	return &todo, nil
 }
 
+// loadTodoTags loads tags for a todo through the polymorphic taggings
+func (s *TodoService) loadTodoTags(todo *models.Todo) error {
+	var tags []models.Tag
+
+	err := database.DB.
+		Joins("JOIN taggings ON taggings.tag_id = tags.id").
+		Where("taggings.taggable_id = ? AND taggings.taggable_type = ?", todo.ID, "Todo").
+		Find(&tags).Error
+
+	if err != nil {
+		return err
+	}
+
+	todo.Tags = tags
+	return nil
+}
+
 // CreateTodo creates a new todo
 func (s *TodoService) CreateTodo(userID uint, req CreateTodoRequest) (*models.Todo, error) {
 	// Verify context belongs to user