Rename AssetItemStatus 'assigned' to 'in_use'

'assigned' implied a specific person always holds the item, but
createAssignment() only requires status=in_stock to hand something out --
the status value itself never guaranteed a matching Assignment row exists.
~158 of the 613 real-imported assets carry this status with no Assignment
at all (shared/unowned equipment like meeting-room gear), so "assigned"
was actively misleading for them. 'in_use' doesn't carry that implication.

DB: `ALTER TYPE "AssetItemStatus" RENAME VALUE 'assigned' TO 'in_use'`
(hand-written migration -- runs cleanly, no data migration needed, every
existing row's status just reads under the new name). Renamed across:
schema.prisma enum, src/lib/assets.ts (type union + IMPORT_STATUS_MAP,
which keeps an `assigned` key mapped to `in_use` for backward-compat with
pre-2026-08-25 exports/CSVs), src/lib/assignments.ts (the literal status
value createAssignment() sets), src/app/assets/page.tsx + api/assets/
route.ts (filter dropdown + validStatuses), src/app/maintenance-records/
page.tsx (its own copy of the status list for the "close" targetStatus
picker). maintenance-records.ts itself needed no logic change -- its
closeableTargetStatusValues was already just ["in_stock", "retired"],
"assigned" was never a valid close target, only mentioned in comments
(updated for accuracy).

Also fixes a real regression found while verifying this: tests/
assets.test.ts (a pre-existing, properly-reviewed test file, not written
this session) broke 2 commits ago when GET /api/assets's signature and
response shape changed for pagination -- it still called GET() with no
Request and expected a bare array. tsc didn't catch this until now
because this container has no dev bind mount (Dockerfile is COPY-based,
`docker exec` only reflects the last build) -- see the
ubi-dan-itam-docker-no-bind-mount memory note. Added a getRequest()
helper and updated assertions to the {items, total} shape.

Verified by actually rebuilding and running the full suite fresh (not
just tsc/lint smoke checks): 369/369 tests pass, including
assignments.test.ts and maintenance-records.test.ts, which this rename
touches directly. tsc/lint clean against the rebuilt image. Migration
applied to the real DB, data verified intact (542 in_use + 71 in_stock =
613, matches pre-rename counts exactly).

tests/maintenance-records.test.ts

@@ -61,7 +61,7 @@ async function createTestModelId(): Promise<string> {
 let serialCounter = 0;
 function createTestAssetItem(overrides: {
   modelId: string;
-  status?: "in_stock" | "assigned" | "repair" | "retired";
+  status?: "in_stock" | "in_use" | "repair" | "retired";
 }) {
   serialCounter += 1;
   return prisma.assetItem.create({
@@ -223,7 +223,7 @@ describe("createMaintenanceRecord", () => {
 
   it("rejects opening a record for an AssetItem that has an active assignment", async () => {
     const modelId = await createTestModelId();
-    const assetItem = await createTestAssetItem({ modelId, status: "assigned" });
+    const assetItem = await createTestAssetItem({ modelId, status: "in_use" });
     const employee = await createTestEmployee();
     await prisma.assignment.create({
       data: { employeeId: employee.id, assetItemId: assetItem.id, assignedAt: new Date() },
@@ -495,7 +495,7 @@ describe("POST /api/maintenance-records", () => {
 
   it("rejects creating a record for an AssetItem with an active assignment", async () => {
     const modelId = await createTestModelId();
-    const assetItem = await createTestAssetItem({ modelId, status: "assigned" });
+    const assetItem = await createTestAssetItem({ modelId, status: "in_use" });
     const employee = await createTestEmployee();
     await prisma.assignment.create({
       data: { employeeId: employee.id, assetItemId: assetItem.id, assignedAt: new Date() },