Add per-user LLM rate limit to /api/ai/triage

[lai3d]

Summary

Adds a second rate-limit layer specifically for LLM-spending requests. The existing global per-IP limiter doesn't fit the cost shape — it protects connection pressure, not token budget — and an admin/operator who's already past RBAC can still spend without bound.

Mechanism

A sliding window keyed on the authenticated user's UUID (CurrentUser.id, stable across both JWT and API-key auth). Implemented in routes::ai_triage::check_llm_rate_limit using the same Redis pattern as the global limiter but with a separate prefix (llm-rate:{user_id}).

Scenario	Behaviour
Within LLM_RATE_LIMIT_REQUESTS per window	200 (and counts toward the next window)
Above limit	429 Too Many Requests with Retry-After: <secs>
Redis unreachable	Fails open (warn log) — availability over perfect accounting

Env var	Default	Purpose
LLM_RATE_LIMIT_REQUESTS	20	Max triages per window, per user
LLM_RATE_LIMIT_WINDOW	3600	Window length in seconds (1 hour)

Independent of the global RATE_LIMIT_REQUESTS / RATE_LIMIT_WINDOW — ten operators behind one NAT no longer share a token budget, and one operator hammering from many IPs can't bypass it.

Why a new AppError variant

Existing handler errors map to 4 status codes; 429 was missing. Added AppError::TooManyRequests { message, retry_after_secs } so the new code path stays in the standard handler return type (Result<Json<T>, AppError>) rather than constructing a Response inline. Cheap, well-tested, and reusable for future endpoints that need 429.

Tests

• 2 new unit tests on AppError::TooManyRequests IntoResponse — 429 status, Retry-After header present/absent.
• 1 new integration test `test_per_user_rate_limit_triggers_429`: uses a new `common::setup_with_llm_limit(2, 60)` helper to pin the limit low, makes 3 requests, verifies the 3rd returns 429 with Retry-After in 1..=60.
• All existing tests (RBAC, parser, provider) untouched and still passing.

Docs

docs/ai-triage.{en,zh}.md gain a "Per-user rate limit" subsection between Auth and OpenAPI.

Test plan

• cargo build --tests — already green locally
• cargo test --lib errors:: — 7/7 green locally (includes 2 new)
• CI runs the new integration test against Redis
• Manual smoke: set LLM_RATE_LIMIT_REQUESTS=2 and confirm the 3rd request returns 429 with Retry-After

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