[P0][AI-02] Prove fail-closed chat quotas and 429 UX without exhausting live allowance #222

Goal

Complete the missing post-release quota/abuse behavior acceptance for the live Persian chatbot while preserving real free-provider capacity. Core chat HTTP 200 was verified; post-release 429 saturation was NOT_RUN.

Scope

• Inspect deployed /api/ai/chat, atomic PostgreSQL quota migration/guard, signed anonymous cookie, rate limits, body/history/token and concurrency bounds. Confirm the actual limits from code; historical documented starting values (5/visitor/day, 2/visitor/minute, 40/site/day, 5/site/minute) are not a substitute for current implementation proof.
• In an isolated local/staging database with mock provider only, test exact threshold successes and rejections, concurrent requests, UTC reset, fail-closed when database unavailable, upstream timeout/429, invalid Origin/CSRF, input invalidity, and Persian user-facing error copy.
• Verify React/UI behavior for 429/503 and retry guidance with browser automation. Real production stress, using many anonymous cookies or exhausting the live quota, is prohibited.
• Optionally perform one bounded safe public read/negative-contract smoke if all dependencies allow and it doesn't invoke/burn provider capacity; record exact evidence, not inferred success. Any real inference needs separate owner-approved limited test budget.
• Produce focused unit/integration/Playwright tests, non-secret report and PR. Release requires a distinct explicit owner approval.

Acceptance

Deterministic test matrix with PASS/FAIL/NOT_RUN for quota, upstream, UI and error boundaries; no actual Cloudflare quota saturation or billing side effects. PROD_429_SATURATION remains NOT_RUN unless specifically approved.

Dependencies

Coordinate with [AI-01] billing attestation; no prod changes until approved. Reference #210 and https://github.com/alirezasafaei-dev/persiantoolbox/blob/main/docs/ai/ONLINE_CHAT_MVP_2026-10-08.md.

Status: READY_FOR_READ_ONLY_AND_ISOLATED_TESTS; production-invasive steps NOT_AUTHORIZED.