Implement annotations and changes

src/models/annotation.go

@@ -0,0 +1,16 @@
+package models
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+)
+
+// Annotation represents drawing annotations on a specific page of a sheet.
+// The primary key is composite: (SheetUuid, Page).
+type Annotation struct {
+	SheetUuid    uuid.UUID `json:"sheetUuid" gorm:"type:uuid;primaryKey"`
+	Page         int       `json:"page" gorm:"primaryKey"`
+	LastModified time.Time `json:"lastModified" gorm:"not null"`
+	Annotations  string    `json:"annotations" gorm:"type:text"` // JSON string of drawing data
+}