fix: return 400 for Zod validation errors instead of 500

[xiaofengzii]

Summary

This PR updates the global error handler to catch ZodError and return 400 with structured issues array instead of generic 500.

Changes

• Catch ZodError in global error handler (apps/api/src/middleware/errorHandler.js)
• Return 400 status code with structured issues array for validation errors
• Update jobController to use parse (throw on error) instead of safeParse
• Add tests for valid and invalid job creation payloads

Testing

• All tests pass: node --test src/tests/*.test.js

/claim SecureBananaLabs#743
Closes SecureBananaLabs#5753