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/widget.go

@@ -0,0 +1,40 @@
+package models
+
+import (
+	"time"
+)
+
+// WidgetType represents the type of widget
+type WidgetType string
+
+const (
+	WidgetTypeBookmark WidgetType = "bookmark"
+	WidgetTypeNotes    WidgetType = "notes"
+	WidgetTypeWeather  WidgetType = "weather"
+)
+
+// Position represents the x,y coordinates of a widget
+type Position struct {
+	X int `dynamodbav:"x" json:"x"`
+	Y int `dynamodbav:"y" json:"y"`
+}
+
+// Size represents the width and height of a widget
+type Size struct {
+	Width  int `dynamodbav:"width" json:"width"`
+	Height int `dynamodbav:"height" json:"height"`
+}
+
+// Widget represents a modular component that displays specific content
+type Widget struct {
+	ID        string                 `dynamodbav:"widget_id" json:"id"`
+	PageID    string                 `dynamodbav:"page_id" json:"page_id"`
+	UserID    string                 `dynamodbav:"user_id" json:"user_id"`
+	Type      WidgetType             `dynamodbav:"type" json:"type"`
+	Title     *string                `dynamodbav:"title,omitempty" json:"title,omitempty"`
+	Position  Position               `dynamodbav:"position" json:"position"`
+	Size      Size                   `dynamodbav:"size" json:"size"`
+	Config    map[string]interface{} `dynamodbav:"config" json:"config"`
+	CreatedAt time.Time              `dynamodbav:"created_at" json:"created_at"`
+	UpdatedAt time.Time              `dynamodbav:"updated_at" json:"updated_at"`
+}