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/api/routes/nodes.py

@@ -4,7 +4,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.api.deps import get_current_user
 from app.db.database import get_db
-from app.db.models import Node
+from app.db.models import Design, Node
 from app.schemas.nodes import NodeCreate, NodeResponse, NodeUpdate
 
 router = APIRouter()
@@ -18,7 +18,15 @@ async def list_nodes(db: AsyncSession = Depends(get_db), _: str = Depends(get_cu
 
 @router.post("", response_model=NodeResponse, status_code=status.HTTP_201_CREATED)
 async def create_node(body: NodeCreate, db: AsyncSession = Depends(get_db), _: str = Depends(get_current_user)) -> Node:
-    node = Node(**body.model_dump())
+    data = body.model_dump()
+    # Attach to a design so the node lands on a canvas. Clients that don't send a
+    # design_id (e.g. the MCP write tools) would otherwise create design_id=null
+    # nodes that exist in the DB but never render in the UI until a container
+    # restart reconciles them. Fall back to the first design, matching bulk-approve.
+    if data.get("design_id") is None:
+        first_design = (await db.execute(select(Design).order_by(Design.created_at).limit(1))).scalar()
+        data["design_id"] = first_design.id if first_design else None
+    node = Node(**data)
     db.add(node)
     await db.commit()
     await db.refresh(node)