Add HTTP API for managing TIDAL favorites

[prietus]

Summary

Implements the favorites HTTP API discussed in #241 (option A — endpoints in
the existing HTTP frontend).

• Registers a Mopidy http:app so the routes mount at /tidal/ alongside
  the rest of the Mopidy HTTP frontend.
• POST /tidal/favorites/<kind>s with body { "id": "<id>" } adds a
  favorite. Returns 204.
• DELETE /tidal/favorites/<kind>s/<id> removes a favorite. Returns 204.
• GET /tidal/favorites/<kind>s lists current favorites as
  [{ id, name, artist? }, ...].
• <kind> is one of album, track, artist, playlist.
• Authentication is implicit: the handlers reach the TidalBackend actor
  via pykka.ActorRegistry and reuse the tidalapi.Session it already
  holds, so clients don't re-implement OAuth.
• Returns 503 if the backend isn't logged in yet, 400 for invalid/empty body
  on POST.

Closes #241.

Test plan

• uv run pytest tests/test_web.py tests/test_extension.py — 31 passing
• uv run ruff format --check — clean
• uv run ruff check — clean
• Smoke-tested against a live Mopidy + mopidy-tidal: POST adds the album
  to the TIDAL library, GET lists it back, DELETE removes it, and the
  changes show up on the official client / web app on next refresh.

[2e0byo]

what is this proxy object? a pykka proxy, right?

[2e0byo]

(I can't approve the workflow.) From a quick glance the code looks good.

I'd question the choice of 503, although it is defensible.

The 503 (Service Unavailable) status code indicates that the server is currently unable to handle the request due to a temporary overload or scheduled maintenance, which will likely be alleviated after some delay. [RFC9110]

Neither of these is true here: it's just that mopidy-tidal is using lazy login and we need you to login. I think you should definitely make sure the error text says something like "please ensure that you can play music from tidal using mopidy-tidal and then retry", and consider raising 403 instead, although the re-auth flow is a bit weird ("go and play a track").

Thanks for the code, this is definitely something it'd be good to have.

[prietus]

Thanks for the review! Based on the feedback (and given the
workflow-approval issue) I'm going to keep this living in a companion package (github.com/prietus/mopidy-tidal-goodies)
rather than push for upstream inclusion — that lets me iterate at my own cadence and keeps mopidy-tidal's scope tighter.

Applied the 503→403 distinction you suggested there (with an actionable error message pointing at the lazy-login flow).
Closing this and deleting the branch.

Thanks again for the time.