feat(drm): let a track pin the DRM system it licenses with

Adds Track.drm_preference so a service can pin a track to Widevine or
PlayReady when a manifest advertises both but only one will license it.
Honoured in DASH/HLS selection, the dl licence path and the remote/API
track round-trip. Also fixes quality-keyed CDM maps sending audio to the
default entry, and a licence callable binding the CDM before the swap.

tests/remote/unit/test_handlers_serialize.py

@@ -7,6 +7,7 @@ from langcodes import Language
 
 from unshackle.core.api.errors import APIError, APIErrorCode
 from unshackle.core.api.handlers import (
+    drm_preference_name,
     sanitize_log,
     search_handler,
     serialize_audio_track,
@@ -161,6 +162,31 @@ def test_serialize_audio_track_basic() -> None:
     assert d["descriptive"] is False
 
 
+def test_serialize_video_track_drm_preference_round_trip() -> None:
+    d = serialize_video_track(video())
+    assert d["drm_preference"] is None
+    v = video()
+    v.drm_preference = "pr"
+    assert serialize_video_track(v)["drm_preference"] == "pr"
+
+
+def test_serialize_audio_track_drm_preference() -> None:
+    a = audio()
+    a.drm_preference = "widevine"
+    assert serialize_audio_track(a)["drm_preference"] == "widevine"
+
+
+def test_drm_preference_name_maps_short_forms() -> None:
+    v = video()
+    assert drm_preference_name(v) is None
+    v.drm_preference = "pr"
+    assert drm_preference_name(v) == "playready"
+    v.drm_preference = "wv"
+    assert drm_preference_name(v) == "widevine"
+    v.drm_preference = "widevine"
+    assert drm_preference_name(v) == "widevine"
+
+
 def test_serialize_subtitle_track_basic() -> None:
     d = serialize_subtitle_track(subtitle(forced=True))
     assert d["id"] == "sub-001"

tests/remote/unit/test_remote_service_helpers.py

@@ -108,6 +108,23 @@ def test_build_tracks_aggregates() -> None:
     assert len(t.subtitles) == 1
 
 
+def test_build_tracks_restores_drm_preference() -> None:
+    data = {
+        "video": [{"id": "v", "codec": "AVC", "width": 1280, "height": 720, "drm_preference": "playready"}],
+        "audio": [{"id": "a", "codec": "AAC", "channels": 2, "drm_preference": "wv"}],
+        "subtitles": [],
+    }
+    t = build_tracks(data)
+    assert t.videos[0].drm_preference == "playready"
+    assert t.audio[0].drm_preference == "wv"
+
+
+def test_build_tracks_ignores_unknown_drm_preference() -> None:
+    data = {"video": [{"id": "v", "codec": "AVC", "width": 1, "height": 1, "drm_preference": "fairplay"}]}
+    t = build_tracks(data)
+    assert t.videos[0].drm_preference is None
+
+
 def test_match_track_by_id() -> None:
     a = deserialize_video({"id": "v1", "codec": "AVC", "width": 1920, "height": 1080})
     b = deserialize_video({"id": "v2", "codec": "AVC", "width": 1280, "height": 720})