Complete tasks 3.2-3.3: Data models and DynamoDB table schemas

- Defined all 8 data models (Page, Widget, Bookmark, Note, TagAssociation, Group, Share, Preferences)
- Implemented DynamoDB table creation for all tables with proper schemas
- Added GSIs for efficient querying (UserBookmarksIndex, UserNotesIndex, TagItemsIndex, UserSharesIndex)
- Comprehensive test coverage for all table schemas
- Updated init-db command to create all tables

internal/models/group.go

@@ -0,0 +1,16 @@
+package models
+
+import (
+	"time"
+)
+
+// Group represents a named collection of bookmarks within a widget
+type Group struct {
+	ID        string    `dynamodbav:"group_id" json:"id"`
+	WidgetID  string    `dynamodbav:"widget_id" json:"widget_id"`
+	UserID    string    `dynamodbav:"user_id" json:"user_id"`
+	Name      string    `dynamodbav:"name" json:"name"`
+	Order     int       `dynamodbav:"order" json:"order"`
+	CreatedAt time.Time `dynamodbav:"created_at" json:"created_at"`
+	UpdatedAt time.Time `dynamodbav:"updated_at" json:"updated_at"`
+}