feat: 无限画布 — infinite canvas with generation integration

Backend:
- canvases table + owner-scoped CRUD under /admin/api/canvas
  (list/create/get/save/delete, 50 docs/user, 4MiB cap, JSON validation)

Frontend:
- /canvas full-screen workspace: DOM+CSS-transform infinite pan/zoom
  (GPU-composited world layer, viewport culling, rAF-batched pointer moves)
- nodes: image (thumb display, natural-size correction), note
  (uncontrolled contenteditable), generation placeholder with edges
- generate on canvas: prompt from panel or selected notes, selected image
  nodes as references, results auto-placed + bezier edges to sources
- multi-canvas management, debounced autosave + keepalive flush,
  snapshot undo/redo (60 steps), marquee/keyboard interactions

backend/internal/bootstrap/app.go

@@ -146,6 +146,8 @@ func NewApp(ctx context.Context) (*App, error) {
 	bannedWordRepo := repo.NewBannedWordRepository(db)
 	v1Svc.SetBannedWords(bannedWordRepo)
 	userGenSvc := service.NewUserGenerationService(v1Svc, eventRepo, userRepo, modelRepo)
+	canvasRepo := repo.NewCanvasRepository(db)
+	canvasSvc := service.NewCanvasService(canvasRepo)
 
 	engine := router.New(cfg, authSvc, router.Handlers{
 		Health:        handler.NewHealthHandler(),
@@ -166,6 +168,7 @@ func NewApp(ctx context.Context) (*App, error) {
 		Announcement:  handler.NewAnnouncementHandler(announcementSvc),
 		Payment:       handler.NewPaymentHandler(paymentSvc),
 		BannedWords:   handler.NewBannedWordsHandler(bannedWordRepo),
+		Canvas:        handler.NewCanvasHandler(canvasSvc),
 	})
 
 	// Background self-healing sweep (quota recovery, cookie refresh, stale-pending