feat(04-01): add categoryId to threads schema, Zod schemas, and test helper

- Add categoryId column with FK to categories on threads table
- Update createThreadSchema to require categoryId
- Update updateThreadSchema to allow optional categoryId
- Update test helper CREATE TABLE to include category_id

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

src/db/schema.ts

@@ -33,6 +33,9 @@ export const threads = sqliteTable("threads", {
   name: text("name").notNull(),
   status: text("status").notNull().default("active"),
   resolvedCandidateId: integer("resolved_candidate_id"),
+  categoryId: integer("category_id")
+    .notNull()
+    .references(() => categories.id),
   createdAt: integer("created_at", { mode: "timestamp" })
     .notNull()
     .$defaultFn(() => new Date()),