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/model/models.go

@@ -250,6 +250,7 @@ func AutoMigrateModels() []any {
 		&StatCounter{},
 		&ConcurrencyGroup{},
 		&Order{},
+		&Canvas{},
 	}
 }
 
@@ -289,6 +290,18 @@ type ConcurrencyGroup struct {
 	UpdatedAt      time.Time
 }
 
+// Canvas is one user's infinite-canvas document. Data stores the whole
+// node/edge graph as a versioned JSON blob; saves are whole-document
+// replacements (last write wins), which matches the single-editor frontend.
+type Canvas struct {
+	ID        string    `gorm:"primaryKey;size:40"`
+	UserID    string    `gorm:"size:32;index;not null"`
+	Title     string    `gorm:"size:120;not null;default:'未命名画布'"`
+	Data      string    `gorm:"type:text"`
+	CreatedAt time.Time
+	UpdatedAt time.Time `gorm:"index"`
+}
+
 // StatCounter is a persistent monotonic counter (key → value), independent of the
 // event_log (which is retention-pruned / clearable). Used for the dashboard
 // cumulative cards (total/success/failed/image/video/api) so they never reset.