Handle async validation errors

[sodalone]

Summary

• add an async route wrapper so rejected async controller promises reach Express error middleware
• return structured 400 JSON responses for Zod validation errors
• add regression coverage for invalid /api/jobs payloads so the request returns validation errors instead of hanging

Validation

• node --test apps\api\src\tests\*.test.js passes
• git diff --check passes

Note: root npm test is fixed separately in #1892; this branch was verified with the direct Node test command above to avoid mixing the two scoped fixes.

Fixes #1893
/claim #743

[sodalone]

Validation transcript from this branch:

$ node --test apps\api\src\tests\*.test.js
pass 8
fail 0

$ git diff --check
(no output)

Coverage note: the new regression test posts an invalid job payload and asserts the API returns HTTP 400 instead of leaving the request unresolved.