feature!: параметр user_agent убран, UA всегда родной; docs/Usage.md

- user_agent в запросе → 422 с подсказкой брать UA из ответа: UA, отличный от реального
  Chrome, расходится с userAgentData и движком, Cloudflare такой браузер не пропускает
  (замер: родной 3/3, Chrome 147 и Windows — 0) (#87)
- профиль хранит только отпечаток (экран, DPR), UA не подменяется (#89)
- docs/Usage.md: практики для клиентов API — тот же UA и прокси, что у браузера, сессии,
  cookies, ошибки и повторы, таймауты, пример на httpx
- README, docs/API.md, docs/Introduction.md; тесты под новый контракт

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

README.md

@@ -38,8 +38,10 @@ Client → Caddy (TLS, on the host) → FastAPI POST /v1/solve → browser slot
   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. User agent and fingerprint (screen, DPR) are
-  fixed on the first request of a session; cookies passed in a request replace the profile's cookies.
+- **Sessions**: `session_id` cookie → persistent Chrome profile on disk. A fingerprint (screen, DPR) 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.
 - **Logging**: stdout, level from `LOG_LEVEL`; client secrets (proxy credentials, 2captcha key, `fill` values,
   cookie values) are masked.
 - **Deployment**: single Docker container; TLS is terminated by Caddy on the host — see
@@ -110,7 +112,8 @@ Response:
 
 On failure `status` is `"err"` with a machine-readable `code` (`no_slots`, `request_failed`, `challenge_failed`,
 `action_failed`) and a human-readable `error`. Full reference — fields, action types, errors, sessions:
-[docs/API.md](docs/API.md).
+[docs/API.md](docs/API.md). How to use the API from your client (same UA and proxy as the browser, sessions,
+retries) — [docs/Usage.md](docs/Usage.md).
 
 ## Roadmap