Add AssetItemStatus.lost, category filter on /assets page

lost added via ALTER TYPE ... ADD VALUE (hand-written migration, stays an
enum -- see the schema.prisma comment on why: status values are read
directly by business logic like createAssignment()s in_stock gate, a
lookup table would not actually decouple that). IMPORT_STATUS_MAP gains
lost/missing -> lost mappings for CSV import/re-sync.

/assets page gets a Category filter dropdown (alongside the existing
Status/Model/search filters) that also narrows the Model dropdown to that
category -- with 135 models after the 2026-08-24 real-data import, an
unfiltered Model list is unwieldy. Picking a Category resets the Model
filter if the currently-selected model does not belong to it.

tsc/lint clean, no Vitest coverage -- same deviation as the rest of this
branch, see CLAUDE.md.

prisma/schema.prisma

@@ -84,14 +84,19 @@ model Employee {
   @@map("employees")
 }
 
-// Fixed, closed set of 4 values tied to the domain (see docs/
+// Fixed, closed set of values tied to the domain (see docs/
 // asset-license-management.md's "Data model (sơ bộ)" table) — native
-// Postgres enum, same rationale as RequestType above.
+// Postgres enum, same rationale as RequestType above. `lost` added
+// 2026-08-25 (ALTER TYPE ... ADD VALUE, not a table -- see the
+// 2026-08-24/25 discussion in CLAUDE.md on why this stays an enum: the
+// values are read directly by business logic, e.g. createAssignment()'s
+// `status !== "in_stock"` gate, not just display labels).
 enum AssetItemStatus {
   in_stock
   assigned
   repair
   retired
+  lost
 }
 
 // Hardware inventory — see docs/asset-license-management.md's "Data model