fix: attach MCP-created nodes/edges to a design (#225)

create_node/create_edge persisted rows with design_id=null when the
client omitted it (the MCP write tools), so they existed in the DB but
never rendered on the canvas until a container restart reconciled them.
Both routes now fall back to the first design, matching bulk-approve.

Also fix MCP resource reads (homelable://canvas, homelable://edges):
the framework passes a pydantic AnyUrl, not a str, which raised
"'AnyUrl' object has no attribute 'startswith'". Coerce to str.

EdgeResponse now exposes design_id for symmetry with NodeResponse.

ha-relevant: no

backend/app/schemas/edges.py

@@ -51,6 +51,7 @@ class EdgeUpdate(BaseModel):
 
 class EdgeResponse(EdgeBase):
     id: str
+    design_id: str | None = None
     created_at: datetime
 
     model_config = {"from_attributes": True}