feat: surface inventory timestamps on nodes

Add creation_date, last_scan and last_modify to the node inventory and the
right-hand detail panel (last_seen already shown):

- creation_date  -> existing created_at column
- last_modify    -> existing updated_at column (bumped on any node change)
- last_scan      -> new column, stamped when a scan observes a node by IP/MAC
- last_seen      -> unchanged

Backend: new nodes.last_scan column + idempotent migration, NodeResponse
field, scanner stamps matching canvas nodes per scanned host. Frontend:
NodeData fields + DetailPanel rows with UTC-safe timestamp formatting.

ha-relevant: maybe

backend/app/schemas/nodes.py

@@ -73,6 +73,7 @@ class NodeResponse(NodeBase):
     design_id: str | None = None
     ieee_address: str | None = None
     last_seen: datetime | None = None
+    last_scan: datetime | None = None
     response_time_ms: int | None = None
     created_at: datetime
     updated_at: datetime