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/http/handler/canvas.go

@@ -0,0 +1,107 @@
+package handler
+
+import (
+	"errors"
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+
+	"backend/internal/model"
+	"backend/internal/service"
+)
+
+// CanvasHandler exposes the infinite-canvas CRUD under the session-authed
+// /admin/api group. Every operation is scoped to the logged-in user.
+type CanvasHandler struct {
+	svc *service.CanvasService
+}
+
+func NewCanvasHandler(svc *service.CanvasService) *CanvasHandler {
+	return &CanvasHandler{svc: svc}
+}
+
+// List serves the canvas picker (id/title/updatedAt only).
+func (h *CanvasHandler) List(c *gin.Context) {
+	items, err := h.svc.List(c.Request.Context(), currentUser(c).ID)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"detail": "failed to list canvases"})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"data": items})
+}
+
+// Create provisions a new canvas; title is optional.
+func (h *CanvasHandler) Create(c *gin.Context) {
+	var body struct {
+	 Title *string `json:"title"`
+	}
+	if err := c.ShouldBindJSON(&body); err != nil {
+		// Empty body is fine — treat as no title.
+		body.Title = nil
+	}
+	cv, err := h.svc.Create(c.Request.Context(), currentUser(c).ID, deref(body.Title))
+	if err != nil {
+		respondCanvasError(c, err)
+		return
+	}
+	c.JSON(http.StatusOK, canvasDoc(cv))
+}
+
+// Get returns the full document blob (parsed by the client).
+func (h *CanvasHandler) Get(c *gin.Context) {
+	cv, err := h.svc.Get(c.Request.Context(), currentUser(c).ID, c.Param("id"))
+	if err != nil {
+		respondCanvasError(c, err)
+		return
+	}
+	c.JSON(http.StatusOK, canvasDoc(cv))
+}
+
+// Save replaces the whole document (autosave) and/or renames.
+func (h *CanvasHandler) Save(c *gin.Context) {
+	var body struct {
+		Title *string `json:"title"`
+		Data  string  `json:"data"`
+	}
+	if err := c.ShouldBindJSON(&body); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"detail": "invalid request body"})
+		return
+	}
+	if err := h.svc.Save(c.Request.Context(), currentUser(c).ID, c.Param("id"), body.Title, body.Data); err != nil {
+		respondCanvasError(c, err)
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"ok": true})
+}
+
+func (h *CanvasHandler) Delete(c *gin.Context) {
+	if err := h.svc.Delete(c.Request.Context(), currentUser(c).ID, c.Param("id")); err != nil {
+		respondCanvasError(c, err)
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"ok": true})
+}
+
+func canvasDoc(cv *model.Canvas) gin.H {
+	return gin.H{"id": cv.ID, "title": cv.Title, "data": cv.Data, "updated_at": cv.UpdatedAt}
+}
+
+func respondCanvasError(c *gin.Context, err error) {
+	switch {
+	case errors.Is(err, service.ErrCanvasNotFound):
+		c.JSON(http.StatusNotFound, gin.H{"detail": err.Error()})
+	case errors.Is(err, service.ErrCanvasLimit),
+		errors.Is(err, service.ErrCanvasTooLarge),
+		errors.Is(err, service.ErrCanvasBadJSON):
+		c.JSON(http.StatusBadRequest, gin.H{"detail": err.Error()})
+	default:
+		c.JSON(http.StatusInternalServerError, gin.H{"detail": "canvas operation failed"})
+	}
+}
+
+func deref(s *string) string {
+	if s == nil {
+		return ""
+	}
+	return *s
+}