feat: show inventory timestamps on Device Inventory tiles

Surface the same lifecycle timestamps on the Device Inventory cards as on the
detail panel. Tiles for devices placed on a canvas show their linked node's
created / last scan / last modified / last seen (correlated by ip or
ieee_address, aggregated across matches: created = oldest, others = newest).
Devices not yet on any canvas fall back to their discovered_at.

Rendered as compact relative times ("2d ago") with the full date on hover, in
a tight two-column footer so the tile keeps its original footprint.

Backend: PendingDeviceResponse gains node_created_at / node_last_scan /
node_last_modified / node_last_seen; the canvas correlation now also pulls node
timestamps in the same single query. Frontend: shared timeFormat util
(absolute + relative), reused by the detail panel.

ha-relevant: maybe

backend/app/schemas/scan.py

@@ -24,6 +24,13 @@ class PendingDeviceResponse(BaseModel):
     # Number of distinct canvases (designs) this device already appears on,
     # correlated by ip / ieee_address against existing nodes. Computed per-request.
     canvas_count: int = 0
+    # Timestamps from the linked canvas node(s), correlated by ip / ieee_address.
+    # Null when the device is not on any canvas yet. Aggregated across matches:
+    # created_at = oldest; last_scan / last_modified / last_seen = newest.
+    node_created_at: datetime | None = None
+    node_last_scan: datetime | None = None
+    node_last_modified: datetime | None = None
+    node_last_seen: datetime | None = None
 
     model_config = {"from_attributes": True}