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/repo/canvas_repo.go

@@ -0,0 +1,84 @@
+package repo
+
+import (
+	"context"
+
+	"gorm.io/gorm"
+
+	"backend/internal/model"
+)
+
+// CanvasRepository persists infinite-canvas documents. All queries are
+// userID-scoped — a canvas is only ever read or written by its owner.
+type CanvasRepository struct {
+	db *gorm.DB
+}
+
+func NewCanvasRepository(db *gorm.DB) *CanvasRepository {
+	return &CanvasRepository{db: db}
+}
+
+// List returns the user's canvases newest-first, without the (potentially
+// large) Data blob — the picker only needs id/title/updatedAt.
+func (r *CanvasRepository) List(ctx context.Context, userID string) ([]model.Canvas, error) {
+	var items []model.Canvas
+	err := r.db.WithContext(ctx).
+		Select("id", "user_id", "title", "updated_at").
+		Where("user_id = ?", userID).
+		Order("updated_at DESC").
+		Find(&items).Error
+	return items, err
+}
+
+func (r *CanvasRepository) Get(ctx context.Context, userID, id string) (*model.Canvas, error) {
+	var c model.Canvas
+	if err := r.db.WithContext(ctx).
+		Where("id = ? AND user_id = ?", id, userID).
+		First(&c).Error; err != nil {
+		return nil, err
+	}
+	return &c, nil
+}
+
+func (r *CanvasRepository) Create(ctx context.Context, c *model.Canvas) error {
+	return r.db.WithContext(ctx).Create(c).Error
+}
+
+// Update replaces title and/or the document blob. A no-match update reports
+// ErrRecordNotFound (gorm's Updates is silent about missing rows).
+func (r *CanvasRepository) Update(ctx context.Context, userID, id string, fields map[string]any) error {
+	res := r.db.WithContext(ctx).
+		Model(&model.Canvas{}).
+		Where("id = ? AND user_id = ?", id, userID).
+		Updates(fields)
+	if res.Error != nil {
+		return res.Error
+	}
+	if res.RowsAffected == 0 {
+		return gorm.ErrRecordNotFound
+	}
+	return nil
+}
+
+func (r *CanvasRepository) Delete(ctx context.Context, userID, id string) error {
+	res := r.db.WithContext(ctx).
+		Where("id = ? AND user_id = ?", id, userID).
+		Delete(&model.Canvas{})
+	if res.Error != nil {
+		return res.Error
+	}
+	if res.RowsAffected == 0 {
+		return gorm.ErrRecordNotFound
+	}
+	return nil
+}
+
+// CountByUser backs the per-user document cap.
+func (r *CanvasRepository) CountByUser(ctx context.Context, userID string) (int64, error) {
+	var n int64
+	err := r.db.WithContext(ctx).
+		Model(&model.Canvas{}).
+		Where("user_id = ?", userID).
+		Count(&n).Error
+	return n, err
+}