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/bookmark.go

@@ -0,0 +1,20 @@
+package models
+
+import (
+	"time"
+)
+
+// Bookmark represents a link with title, URL, and optional grouping
+type Bookmark struct {
+	ID         string    `dynamodbav:"bookmark_id" json:"id"`
+	WidgetID   string    `dynamodbav:"widget_id" json:"widget_id"`
+	UserID     string    `dynamodbav:"user_id" json:"user_id"`
+	Title      string    `dynamodbav:"title" json:"title"`
+	URL        string    `dynamodbav:"url" json:"url"`
+	GroupID    *string   `dynamodbav:"group_id,omitempty" json:"group_id,omitempty"`
+	Order      int       `dynamodbav:"order" json:"order"`
+	FaviconURL *string   `dynamodbav:"favicon_url,omitempty" json:"favicon_url,omitempty"`
+	CreatedAt  time.Time `dynamodbav:"created_at" json:"created_at"`
+	UpdatedAt  time.Time `dynamodbav:"updated_at" json:"updated_at"`
+	Version    int       `dynamodbav:"version" json:"version"`
+}