Add scripts/rate_limit.py with cloud-aware key function:
- In cloud mode, extracts user_id from _request_db ContextVar path (part[-3])
so each cloud user has their own rate limit bucket
- In demo mode, returns unique per-request key to disable limiting entirely
(_demo_guard handles write-blocking; rate limiting would block the demo UX)
- Falls back to client IP for local/self-hosted installs
Wire limiter to 4 endpoints with conservative per-user limits:
- POST /generate/cover-letter: 20/hour
- POST /research/run: 10/hour
- POST /qa/suggest: 60/hour
- POST /survey/analyze: 30/hour
Add _demo_guard() to generate_research and suggest_qa_answer (was missing).
Fix pre-existing silent except in suggest_qa_answer: was bare except pass,
now logs warning with exc_info.
Add _RL_WIZARD placeholder constant with TODO to wire to wizard/ai/interview
after feat/77 merges (declared but intentionally not applied yet to avoid
false sense of security — comment makes the gap explicit).
18 tests covering cloud user isolation, demo bypass, IP fallback, all 4
endpoints returning 429 on excess, retry_after header, and demo guard.
Closes: #122
