fix: solve the s.to captcha modal when a mirror answers the redirect

[mzmonkey]

🏷️ Title Format

• My PR title starts with one of the prefixes above.
• Everything I wrote here is in English.

📝 Description

• SerienstreamEpisode.provider_url decides whether the provider redirect needs the captcha modal by comparing the response host with the redirect URL's host: a different host is taken as "the redirect left s.to".
• The mirror failover (mirrors.request_with_failover) can answer from another s.to mirror than the one the redirect was built on:
  • s.to currently doesn't resolve (NXDOMAIN), so every primary retry fails over to serienstream.to;
  • after a hiccup on serienstream.to the failover moves to the bare-IP mirror 186.2.175.5 and stays there for _PRIMARY_RETRY_AFTER.
• In both cases the host differs, so the s.to page itself (captcha modal included) is handed to the provider extractor. solve_sto_modal is never called, and every episode fails within about a second with Keine VOE-Videoquelle auf der Seite gefunden. until the failover switches back.
• The check now uses mirrors.site_for_url(resp.url) != "sto": only a response outside every s.to mirror counts as having reached the provider. Everything else goes through solve_sto_modal as intended.

Impact seen on a 1.6.0 install: roughly 300 failed episodes per hour across several S.TO series for a full day. AniWorld, filmo and megakino were unaffected. With this change applied, the next episodes all downloaded and none failed.

🔗 Related Issue(s)

• None filed; found while debugging the failures above.

🔄 Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)

🧪 How Has This Been Tested?

• I have run the application locally (via Python or Docker). (1.6.0 with the same change applied)
• I have verified that media downloading still works as intended (if applicable).
• I have verified that the WebUI renders correctly without errors (if applicable). (no UI change)

New tests/test_sto_provider_redirect.py: a redirect that reached the provider is taken as is; the bare-IP mirror, a dead s.to failing over to serienstream.to, and the same mirror all still get the modal solved. Without the fix, the two mirror cases fail. tests/test_providers.py and tests/test_infra_mirrors.py still pass.

✅ Checklist

• I have performed a self-review of my own code.
• My code does not introduce new warnings or errors (e.g., Python syntax).
• Code comments are written in English.

Diagnosed and written with the help of Claude Code; reviewed and tested on my own 1.6.0 install.

[TheMRX13]

Will be checked today