Fix Spotify metadata provider: send token via Authorization header

[nicobgr]

Fixes #467

Problem

Spotify's Web API no longer accepts access_token as a query parameter on /v1/search - it must be sent as an Authorization: Bearer <token> header instead. Requests built the old way get rejected with an error-shaped JSON response that handle_json_result_error doesn't expect (no tracks/albums/artists key), which crashes with an unhandled TypeError.

Since thirdparty/__init__.py's get_image_*_all functions log e.__doc__ (the exception class's docstring) instead of the actual error, this surfaces in the logs as the generic, undiagnosable:

[thirdparty] Error getting album image from Spotify: Inappropriate argument type.

...on every single lookup (verified: 0/163 successful Spotify lookups in a real instance's logs before this fix). It silently falls through to the next provider, so it's easy to miss - the only symptom is worse-than-expected cover art coverage.

I believe this also explains #424 ("Mapping key not found" = KeyError.__doc__, same class of problem, different exception).

Fix

• maloja/thirdparty/spotify.py: override get_image_track/get_image_artist/get_image_album to send the token via the Authorization header instead of the URL, drop access_token={token} from the URL templates, and make handle_json_result_error defensive against non-dict / error-shaped responses instead of crashing on them.
• maloja/thirdparty/__init__.py: log repr(e) instead of e.__doc__ in the three get_image_*_all functions, so future failures (for any provider, not just Spotify) are actually diagnosable from the logs instead of always showing the same generic exception-class docstring.

Testing

Verified against a real Maloja instance (v3.2.6) with valid Spotify credentials:

• Before: 0 successful Spotify image lookups across ~160 real requests, all logging the generic TypeError message.
• After: confirmed real i.scdn.co image URLs returned for artist/album/track lookups that previously failed, including via real scrobbles from a live client (not just synthetic tests), e.g. [thirdparty] Got album image for (['Sia'], '1000 Forms Of Fear') from Spotify.