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>

tests/helpers/db.ts

@@ -37,6 +37,7 @@ export function createTestDb() {
       name TEXT NOT NULL,
       status TEXT NOT NULL DEFAULT 'active',
       resolved_candidate_id INTEGER,
+      category_id INTEGER NOT NULL REFERENCES categories(id),
       created_at INTEGER NOT NULL DEFAULT (unixepoch()),
       updated_at INTEGER NOT NULL DEFAULT (unixepoch())
     )