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/tag.go

@@ -0,0 +1,29 @@
+package models
+
+import (
+	"time"
+)
+
+// ItemType represents the type of item that can be tagged
+type ItemType string
+
+const (
+	ItemTypeBookmark ItemType = "bookmark"
+	ItemTypeNote     ItemType = "note"
+)
+
+// TagAssociation represents a many-to-many relationship between tags and items
+type TagAssociation struct {
+	ItemID    string    `dynamodbav:"item_id" json:"item_id"`
+	TagName   string    `dynamodbav:"tag_name" json:"tag_name"`
+	UserID    string    `dynamodbav:"user_id" json:"user_id"`
+	ItemType  ItemType  `dynamodbav:"item_type" json:"item_type"`
+	CreatedAt time.Time `dynamodbav:"created_at" json:"created_at"`
+}
+
+// TagInfo represents tag metadata with usage statistics
+type TagInfo struct {
+	Name      string     `json:"name"`
+	Count     int        `json:"count"`
+	ItemTypes []ItemType `json:"item_types"`
+}