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/preferences.go

@@ -0,0 +1,22 @@
+package models
+
+import (
+	"time"
+)
+
+// SearchProvider represents the configured search engine
+type SearchProvider string
+
+const (
+	SearchProviderGoogle     SearchProvider = "google"
+	SearchProviderDuckDuckGo SearchProvider = "duckduckgo"
+	SearchProviderBing       SearchProvider = "bing"
+)
+
+// Preferences represents user preferences and settings
+type Preferences struct {
+	UserID         string         `dynamodbav:"user_id" json:"user_id"`
+	SearchProvider SearchProvider `dynamodbav:"search_provider" json:"search_provider"`
+	Theme          *string        `dynamodbav:"theme,omitempty" json:"theme,omitempty"`
+	UpdatedAt      time.Time      `dynamodbav:"updated_at" json:"updated_at"`
+}