fix(rack): stamp the linked node's last-seen with an offset

SQLite has no timezone type: an aware `last_seen` reads back naive, and
Pydantic then put it on the wire as `2026-08-08T10:00:00`. To the browser
that is *local* time, so the Logical view panel printed a timestamp
shifted by the viewer's UTC offset.

ha-relevant: no

backend/app/schemas/racks.py

@@ -1,4 +1,4 @@
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Any
 
 from pydantic import BaseModel, field_validator, model_validator
@@ -283,6 +283,20 @@ class RackInventoryItem(BaseModel):
     # True when this device is already mounted somewhere in this design.
     racked: bool = False
 
+    @field_validator("node_last_seen")
+    @classmethod
+    def _utc(cls, v: datetime | None) -> datetime | None:
+        """Stamp UTC on a naive timestamp before it goes on the wire.
+
+        SQLite has no timezone type: an aware datetime written to `last_seen`
+        reads back naive, and Pydantic then serializes it without an offset.
+        `new Date("2026-08-08T10:00:00")` in the browser is *local* time, so the
+        rack printed a last-seen shifted by the viewer's offset.
+        """
+        if v is not None and v.tzinfo is None:
+            return v.replace(tzinfo=timezone.utc)
+        return v
+
 
 class RackInventoryResponse(BaseModel):
     items: list[RackInventoryItem]

backend/tests/test_racks.py

@@ -1,5 +1,6 @@
 import pytest
 from httpx import AsyncClient
+from sqlalchemy import text
 
 from app.db.models import PendingDevice
 
@@ -512,6 +513,31 @@ class TestInventory:
         assert entry["node_design_id"] == network
         assert entry["node_design_name"] == "Network"
 
+    async def test_stamps_last_seen_with_an_offset(self, client: AsyncClient, headers, db_session):
+        """SQLite reads `last_seen` back naive, and a naive ISO string is *local*
+        time to `new Date()` — the panel printed it shifted by the viewer's offset.
+        """
+        design_id = await _design(client, headers)
+        node = (
+            await client.post(
+                "/api/v1/nodes",
+                json={"type": "server", "label": "seen", "ip": "192.168.1.77"},
+                headers=headers,
+            )
+        ).json()
+        db_session.add(PendingDevice(id="pd-seen", ip="192.168.1.77", suggested_type="server"))
+        await db_session.execute(
+            text("UPDATE nodes SET last_seen = :ts WHERE id = :id"),
+            {"ts": "2026-08-08 10:00:00.000000", "id": node["id"]},
+        )
+        await db_session.commit()
+
+        items = (
+            await client.get(f"/api/v1/racks/inventory?design_id={design_id}", headers=headers)
+        ).json()["items"]
+        entry = next(i for i in items if i["id"] == "pd-seen")
+        assert entry["node_last_seen"].endswith(("Z", "+00:00"))
+
     async def test_leaves_the_node_fields_empty_without_a_match(
         self, client: AsyncClient, headers
     ):