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/database.py

@@ -293,6 +293,9 @@ async def init_db() -> None:
                     "UPDATE nodes SET properties = ? WHERE id = ?",
                     (_json.dumps(props), node_id),
                 )
+        # Inventory timestamp: last time a scan observed this node (idempotent)
+        with suppress(OperationalError):
+            await conn.exec_driver_sql("ALTER TABLE nodes ADD COLUMN last_scan DATETIME")
         # Migrate animated column from boolean (0/1) to string ('none'/'snake')
         with suppress(OperationalError):
             await conn.exec_driver_sql("UPDATE edges SET animated = 'snake' WHERE animated = '1' OR animated = 1")

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)

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

backend/app/services/scanner.py

@@ -11,10 +11,10 @@ from dataclasses import dataclass, field
 from datetime import datetime, timezone
 from typing import Any
 
-from sqlalchemy import select
+from sqlalchemy import or_, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.db.models import PendingDevice, ScanRun
+from app.db.models import Node, PendingDevice, ScanRun
 from app.services.fingerprint import fingerprint_ports, suggest_node_type
 from app.services.http_probe import probe_open_ports
 
@@ -542,6 +542,20 @@ async def run_scan(
                 ))
                 devices_found += 1
 
+            # Stamp last_scan on any canvas node that matches this device by IP
+            # (or MAC, when known) so the inventory shows when the scanner last
+            # observed it. Matches across designs.
+            host_mac = host.get("mac")
+            node_match = [Node.ip == ip]
+            if host_mac:
+                node_match.append(Node.mac == host_mac)
+            matching_nodes = (await db.execute(
+                select(Node).where(or_(*node_match))
+            )).scalars().all()
+            scanned_at = datetime.now(timezone.utc)
+            for node in matching_nodes:
+                node.last_scan = scanned_at
+
             await db.commit()
             await broadcast_scan_update(run_id=run_id, devices_found=devices_found)