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/db/models.py

@@ -61,6 +61,7 @@ class Node(Base):
     bottom_handles: Mapped[int] = mapped_column(Integer, default=1)
     ieee_address: Mapped[str | None] = mapped_column(String, index=True, nullable=True)
     last_seen: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    last_scan: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
     response_time_ms: Mapped[int | None] = mapped_column(Integer)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=_now)
     updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=_now, onupdate=_now)