fix: declare transient node timestamp attrs on PendingDevice

mypy flagged the per-request node_* timestamp attributes as undefined on the
model. Declare them as class-level defaults (like canvas_count) and set
__allow_unmapped__ so SQLAlchemy 2.0 doesn't try to map the Optional[datetime]
annotations as columns.

backend/app/db/models.py

@@ -100,6 +100,9 @@ class CanvasState(Base):
 
 class PendingDevice(Base):
     __tablename__ = "pending_devices"
+    # Permit the plain (non-Mapped[]) annotations on the transient request-only
+    # attributes below; without this SQLAlchemy 2.0 tries to map them as columns.
+    __allow_unmapped__ = True
 
     id: Mapped[str] = mapped_column(String, primary_key=True, default=_uuid)
     ip: Mapped[str | None] = mapped_column(String, nullable=True)
@@ -121,6 +124,12 @@ class PendingDevice(Base):
     # Transient (not persisted): populated per-request by the scan routes to report
     # how many canvases this device already appears on. Not a mapped column.
     canvas_count: int = 0
+    # Transient (not persisted): timestamps from the linked canvas node(s),
+    # correlated by ip / ieee_address. None when the device is not on any canvas.
+    node_created_at: datetime | None = None
+    node_last_scan: datetime | None = None
+    node_last_modified: datetime | None = None
+    node_last_seen: datetime | None = None
 
 
 class PendingDeviceLink(Base):