fix(canvas): 画布版本 - 合并 PR #528 防止旧内容覆盖并支持历史恢复

将画布 revision CAS 与历史快照接到 schema v23，保留当前顶栏积分弹窗和已有菜单入口。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/internal/database/migrations.go

@@ -11,7 +11,7 @@ import (
 	"gorm.io/gorm"
 )
 
-const CurrentSchemaVersion int64 = 22
+const CurrentSchemaVersion int64 = 23
 
 const baselineSchemaChecksum = "sha256:open-ai-canvas-schema-v1-20260830"
 const schemaMigrationAppliedAtIndexChecksum = "sha256:schema-migrations-applied-at-index-v2-20260830"
@@ -86,6 +86,9 @@ var schemaMigrations = []migration{
 	{version: 22, name: "banner_announcement_notice_type", checksum: "sha256:banner-announcement-notice-type-v22-20260917", apply: func(tx *gorm.DB) error {
 		return tx.AutoMigrate(&model.BannerAnnouncement{})
 	}},
+	{version: 23, name: "canvas_revision_history", checksum: "sha256:canvas-revision-history-v23-20260918", apply: func(tx *gorm.DB) error {
+		return tx.AutoMigrate(&model.CanvasProject{}, &model.CanvasSnapshot{}, &model.CanvasSnapshotResource{})
+	}},
 }
 
 func migrateSchemaV14(tx *gorm.DB) error {