feature: шум canvas с seed профиля — отпечатки профилей различаются (#89)

Минимальный init-script (src/api/js/canvas_noise.js): младший бит у ~1/64 непрозрачных
пикселей при toDataURL/toBlob/getImageData, детерминирован от seed профиля. Пустой
canvas не меняется, подменённые функции неотличимы по toString и length. Старые
профили получают seed, экран сохраняется.

Проверено: 4 профиля — 4 разных хеша canvas; Cloudflare 6/6 на новых профилях.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -40,7 +40,7 @@ Client → Caddy (TLS) → api (FastAPI) ──task──▶ RabbitMQ ──▶
   reCAPTCHA, hCaptcha) are solved via the client's 2captcha key.
 - **Proxies**: not managed by the service — the client may supply their own proxy per request. Browser locale and
   timezone are derived from the proxy's exit IP.
-- **Sessions**: `session_id` cookie → persistent Chrome profile on disk, removed after `PROFILE_TTL_DAYS` (7) unused days. A fingerprint (screen, DPR) is generated on
+- **Sessions**: `session_id` cookie → persistent Chrome profile on disk, removed after `PROFILE_TTL_DAYS` (7) unused days. A fingerprint (screen, DPR, canvas noise seed) is generated on
   the first request of a session; cookies passed in a request replace the profile's cookies.
 - **Browser identity is not configurable**: user agent and window size are the real browser's own — a spoofed UA
   mismatches `userAgentData` and Cloudflare blocks it. Clients take `user_agent` from the response.