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/note.go

@@ -0,0 +1,31 @@
+package models
+
+import (
+	"time"
+)
+
+// ContentFormat represents the format mode of note content
+type ContentFormat string
+
+const (
+	ContentFormatPlain    ContentFormat = "plain"
+	ContentFormatRTF      ContentFormat = "rtf"
+	ContentFormatCode     ContentFormat = "code"
+	ContentFormatYAML     ContentFormat = "yaml"
+	ContentFormatMarkdown ContentFormat = "markdown"
+)
+
+// Note represents a text note with rich content support
+type Note struct {
+	ID              string        `dynamodbav:"note_id" json:"id"`
+	WidgetID        string        `dynamodbav:"widget_id" json:"widget_id"`
+	UserID          string        `dynamodbav:"user_id" json:"user_id"`
+	Title           *string       `dynamodbav:"title,omitempty" json:"title,omitempty"`
+	Content         string        `dynamodbav:"content" json:"content"`
+	ContentLocation string        `dynamodbav:"content_location" json:"content_location"`
+	Format          ContentFormat `dynamodbav:"format" json:"format"`
+	Language        *string       `dynamodbav:"language,omitempty" json:"language,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"`
+}