feature: UA и отпечаток задаются профилю один раз, cookies запроса заменяют профильные

- user_agent записывается в профиль при первом запросе сессии, дальше игнорируется (#87)
- переданные cookies заменяют cookies профиля (clear_cookies перед add_cookies) (#88)
- без user_agent профилю генерируется отпечаток browserforge (Linux Chrome): UA с версией
  установленного Chrome, экран и devicePixelRatio; только нативная эмуляция, без JS (#89)
- make fp-check: проверка, что отпечаток применяется и стабилен, и доля различий профилей

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

README.md

@@ -38,12 +38,12 @@ 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 (cookies, local storage, user agent) on disk.
+- **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.
 - **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
   [docs/Deployment.md](docs/Deployment.md).
-- **Canvas fingerprint** is generated anew for every session by design.
 
 ### Project layout
 
@@ -51,6 +51,7 @@ Client → Caddy (TLS, on the host) → FastAPI POST /v1/solve → browser slot
 src/
   main.py              # FastAPI app: /v1/solve, /health, slots, sessions, response
   api/
+    profile.py         # per-profile user agent and generated fingerprint
     schemas.py         # SolveRequest / ActionRequest / SolveResponse + validation
     actions.py         # action registry and execution
     geoip.py           # proxy → locale / timezone
@@ -67,6 +68,7 @@ src/
 docs/                  # API, deployment, anti-bot notes
 tests/
   loadtest.py          # concurrent load test + docker stats
+  fingerprint_check.py # profile fingerprints are applied and stable (make fp-check)
   test_main.http       # manual requests (JetBrains HTTP client)
 wheels/                # wheels for offline install on Ubuntu Noble
 Dockerfile