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/migrations/20260825073402_add_asset_item_status_lost/migration.sql

@@ -0,0 +1,2 @@
+-- Hand-written (same deviation as other 2026-08-2x migrations -- see CLAUDE.md).
+ALTER TYPE "AssetItemStatus" ADD VALUE 'lost';