fix: optimize test infrastructure and fix missing brand migration

- Share PGlite instance per test file (TRUNCATE RESTART IDENTITY instead
  of creating new instance per test) — tests run in ~9s vs minutes
- Add missing 'brand' column to items table migration
- Fix corrupt 0002 snapshot (merge conflict artifacts)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

drizzle-pg/0002_wakeful_vermin.sql

@@ -13,6 +13,7 @@ CREATE TABLE "global_item_tags" (
 --> statement-breakpoint
 ALTER TABLE "items" ADD COLUMN "global_item_id" integer;--> statement-breakpoint
 ALTER TABLE "items" ADD COLUMN "purchase_price_cents" integer;--> statement-breakpoint
+ALTER TABLE "items" ADD COLUMN "brand" text;--> statement-breakpoint
 ALTER TABLE "thread_candidates" ADD COLUMN "global_item_id" integer;--> statement-breakpoint
 UPDATE "items" SET "global_item_id" = (
   SELECT "global_item_id" FROM "item_global_links"