Resolve "Provide a default format when transcoding via Subsonic"

Closes #867 (closed)

Basically, it enforces a default MP3 format (but configurable via the SUBSONIC_DEFAULT_TRANSCODING_FORMAT env var) when: the maxBirate parameter is passed AND != 0 AND the format parameter is missing or empty.

cc @funkwhale/reviewers-python

api/config/settings/common.py

@@ -658,3 +658,6 @@ RSA_KEY_SIZE = 2048
 CREATE_IMAGE_THUMBNAILS = env.bool("CREATE_IMAGE_THUMBNAILS", default=True)
 # we rotate actor keys at most every two days by default
 ACTOR_KEY_ROTATION_DELAY = env.int("ACTOR_KEY_ROTATION_DELAY", default=3600 * 48)
+SUBSONIC_DEFAULT_TRANSCODING_FORMAT = (
+    env("SUBSONIC_DEFAULT_TRANSCODING_FORMAT", default="mp3") or None
+)