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/share.go

@@ -0,0 +1,31 @@
+package models
+
+import (
+	"time"
+)
+
+// Share represents a shareable link for bookmarks or notes
+type Share struct {
+	ID          string     `dynamodbav:"share_id" json:"id"`
+	UserID      string     `dynamodbav:"user_id" json:"user_id"`
+	ItemID      string     `dynamodbav:"item_id" json:"item_id"`
+	ItemType    ItemType   `dynamodbav:"item_type" json:"item_type"`
+	CreatedAt   time.Time  `dynamodbav:"created_at" json:"created_at"`
+	ExpiresAt   *time.Time `dynamodbav:"expires_at,omitempty" json:"expires_at,omitempty"`
+	AccessCount int        `dynamodbav:"access_count" json:"access_count"`
+}
+
+// ShareLink represents a shareable link with full URL
+type ShareLink struct {
+	ShareID   string     `json:"share_id"`
+	URL       string     `json:"url"`
+	ExpiresAt *time.Time `json:"expires_at,omitempty"`
+}
+
+// SharedItem represents the data returned when accessing a shared link
+type SharedItem struct {
+	ItemType   ItemType    `json:"item_type"`
+	Data       interface{} `json:"data"`
+	OwnerEmail *string     `json:"owner_email,omitempty"`
+	CreatedAt  time.Time   `json:"created_at"`
+}