[codex] Add mock interview practice

[leini8891]

Summary

• Adds a conversational mock interview flow from saved applications, JD context, and knowledge-base matches.
• Uses OpenAI when configured and falls back to deterministic playbook questions/feedback when no key is available.
• Saves completed mock sessions into existing interview_records as transcript notes.

Validation

• pnpm lint && pnpm test && pnpm build