Add app_settings migration, settings UI, and OpenAI-compatible httpx LLM client with mocked tests.
Preserve API keys on blank form submissions, require a fresh key when base_url changes, and keep AI search settings untouched for step 3.
Update docs/design LLM integration and step 3 AI search notes, including prompt contract and extra-hints planning.
Add docs/: a bilingual repository brief, plus docs/design/ with the high-level design (Alembic migration foundation, LLM integration, basic AI search) and a self-contained per-step implementation plan (step 1-3).
tliu93