🐛 Fix Cantook crash on Android

[aaronleopold]

This PR contains two (and potentially 3) fixes relating to using the experimental OPDS V2 with Cantook on Android:

1. The id field in the auth document is now the full service URL, not the relative path. This caused an immediate crash on Android for whatever reason
2. The route handler which serves the auth document is now excluded from any auth guard. I am personally not a huge fan of this, as I like to minimize the surface of unauthenticated endpoints. However, there seems to be no other way around it
3. Instances of APIError (and a few other OPDS-specific 401 responses) will attempt to instruct the client to remove any cookies. The former used to exist, however seems to have been accidentally removed as of ♻️ Update axum and refactor middlewares #400

I want to note that none of these issues surfaced in the Cantook iOS app. In fact, wrt the second fix mentioned above, iOS correctly handles the 401 response which contains the auth document whereas the Android app doesn't 🤷

See discord thread for more context

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
apps/server/src/routers/opds/v2_0.rs	0.00%	12 Missing ⚠️
apps/server/src/routers/api/v1/media.rs	0.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
apps/server/src/config/session/utils.rs	29.03% <100.00%> (+29.03%)	⬆️
apps/server/src/errors.rs	26.57% <100.00%> (+4.93%)	⬆️
apps/server/src/middleware/auth.rs	94.36% <100.00%> (+0.08%)	⬆️
apps/server/src/routers/api/v1/media.rs	0.00% <0.00%> (ø)
apps/server/src/routers/opds/v2_0.rs	0.00% <0.00%> (ø)