feat(server): add layer-separation generator task

Introduce the `layer-separation` generator (edit-elements node action)
that splits a source image into a clean background layer, foreground
image artifacts with bounding boxes, and editable text render data.

- New layer_separation.go builds the separated result and artifacts
- SubmitGeneratorTask routes layer-separation and move-object via a
  switch; standalone task completion + polling support
- Extend artifact metadata with BBox and GeneratorName across the
  domain, API types, mapper, .api schema, and API.md
- edit-elements gets a transparent manual-frame placeholder holding the
  separated foreground images and text layers beside the original

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/types/types.go

@@ -417,14 +417,16 @@ type GeneratorTaskArtifact struct {
 }
 
 type GeneratorTaskArtifactMetadata struct {
-	ArtifactId string `json:"artifact_id,optional"`
-	Format     string `json:"format,optional"`
-	Height     int64  `json:"height,optional"`
-	Label      string `json:"label,optional"`
-	NodeId     string `json:"node_id,optional"`
-	NodeName   string `json:"node_name,optional"`
-	SizeBytes  int64  `json:"size_bytes,optional"`
-	Width      int64  `json:"width,optional"`
+	ArtifactId    string  `json:"artifact_id,optional"`
+	BBox          []int64 `json:"bbox,optional"`
+	Format        string  `json:"format,optional"`
+	GeneratorName string  `json:"generator_name,optional"`
+	Height        int64   `json:"height,optional"`
+	Label         string  `json:"label,optional"`
+	NodeId        string  `json:"node_id,optional"`
+	NodeName      string  `json:"node_name,optional"`
+	SizeBytes     int64   `json:"size_bytes,optional"`
+	Width         int64   `json:"width,optional"`
 }
 
 type GeneratorTaskData struct {