test: harden ky create test and cover delete/list routes

[whitewofe]

Summary

/claim #2

Builds on the prior ky-migration test work by removing the remaining race in the create test and filling in coverage for the two things routes that had no tests.

tests/routes/things/create.test.ts

• Awaits the ky.post("things/create", …) call so the create completes before things/list runs (previously fire-and-forget, the assertion only passed by accident on fast paths).
• Asserts the create response body equals { ok: true }.
• After listing, verifies the persisted name, description, and that thing_id is a non-empty string — covers the full create → persist → list contract.

tests/routes/things/delete.test.ts (new)

• Creates two things, deletes one by thing_id via URL-encoded form data, asserts { ok: true } and that only the surviving thing remains.
• Adds a no-op case: deleting a non-existent thing_id still returns { ok: true } and leaves existing data untouched.

tests/routes/things/list.test.ts (new)

• Empty-list case returns { things: [] }.
• Multi-create case verifies things come back in insertion order and that thing_id values are unique and monotonically increasing (matches the idCounter behavior in lib/db/db-client.ts).

Validation

$ bun test
 6 pass
 0 fail
 21 expect() calls

$ bun run format:check
 Checked 24 files in 12ms. No fixes applied.

No production code changed; tests only.

Closes #2