test: await ky create request

[snkk2x-collab]

Summary

• Await the ky-backed things/create POST in the create route regression test.
• Assert the { ok: true } JSON response before listing things.
• Verify the persisted thing fields so the create/list sequence is deterministic.

Validation

• bun test tests/routes/things/create.test.ts
• bun test tests/routes/health.test.ts
• bun run format:check
• git diff --check

Closes #2
/claim #2