fix: save button now fully persists canvas to DB

Root cause: handleAddNode/handleEdgeConfirm only updated Zustand store,
never creating records in the DB. canvasApi.save() only updated positions
of existing nodes, so nothing survived a page refresh.

Fix: save now sends the full canvas state (all nodes + edges + data) and
the backend does a full sync — upsert incoming, delete anything removed.
This means Save is the single source of truth: no need to call individual
create/update/delete APIs for every drag or edit.

backend/app/schemas/canvas.py

@@ -1,17 +1,42 @@
+from typing import Any
+
 from pydantic import BaseModel
 
 from app.schemas.edges import EdgeResponse
 from app.schemas.nodes import NodeResponse
 
 
-class NodePosition(BaseModel):
+class NodeSave(BaseModel):
     id: str
-    x: float
-    y: float
+    type: str
+    label: str
+    hostname: str | None = None
+    ip: str | None = None
+    mac: str | None = None
+    os: str | None = None
+    status: str = "unknown"
+    check_method: str | None = None
+    check_target: str | None = None
+    services: list[Any] = []
+    notes: str | None = None
+    parent_id: str | None = None
+    pos_x: float = 0
+    pos_y: float = 0
+
+
+class EdgeSave(BaseModel):
+    id: str
+    source: str
+    target: str
+    type: str = "ethernet"
+    label: str | None = None
+    vlan_id: int | None = None
+    speed: str | None = None
 
 
 class CanvasSaveRequest(BaseModel):
-    node_positions: list[NodePosition] = []
+    nodes: list[NodeSave] = []
+    edges: list[EdgeSave] = []
     viewport: dict = {}