feat(canvas): 画布同步 - 新增操作事务、预演保存与并发恢复能力

backend/internal/model/models_canvas_sync.go

@@ -0,0 +1,18 @@
+package model
+
+import "time"
+
+// CanvasMutationReceipt makes a client retry observable without making the
+// canvas document carry an ever-growing list of request IDs. The receipt is a
+// recovery hint; the mutation itself remains idempotent because applying the
+// same desired values is a no-op.
+type CanvasMutationReceipt struct {
+	ID            string    `json:"id" gorm:"primaryKey;size:80"`
+	UserID        string    `json:"userId" gorm:"index;size:36;uniqueIndex:idx_canvas_mutation_receipt_identity,priority:1"`
+	CanvasID      string    `json:"canvasId" gorm:"index;size:80;uniqueIndex:idx_canvas_mutation_receipt_identity,priority:2"`
+	MutationID    string    `json:"mutationId" gorm:"size:160;uniqueIndex:idx_canvas_mutation_receipt_identity,priority:3"`
+	PayloadDigest string    `json:"payloadDigest" gorm:"size:64;index"`
+	ResultJSON    string    `json:"resultJson" gorm:"type:text"`
+	Revision      int64     `json:"revision"`
+	CreatedAt     time.Time `json:"createdAt" gorm:"index"`
+}