fix: index albums through the endpoint Lidarr does not throw from

Indexing fetched albums one artist at a time, and `GET /api/v1/album?artistId=`
is Lidarr's unguarded path. It maps straight from the album service with no
hydration: the mapper then dereferences model.Images and model.SecondaryTypes
without a null check, follows model.Artist?.Value where only the first link is
guarded, and selects the monitored release with SingleOrDefault, which throws
outright when an album has two of them. Any of those is a 500 that aborts the
whole index.

The unfiltered `GET /api/v1/album` builds its own artist and release lookups and
skips an album whose metadata is missing rather than dereferencing it. Use that
instead, once, and group by artistId locally. It is the defensive path and it
costs N fewer requests.

Tracks and files have no unfiltered endpoint -- Lidarr rejects a call with no
filter at all -- so those stay per artist. A failure on one artist now skips
that artist rather than ending the run, but the count is recorded in the store
and the coverage report leads with it: a missing artist makes their played music
look unplayed, which is precisely the error that costs music later, so an
incomplete index must not be culled against.

Errors now carry the request URL and whatever the server put in the body. The
original report of this failure was "album: HTTP 500", which points at the URL
and the credentials -- neither of which was at fault.

tests/conftest.py

@@ -1,6 +1,8 @@
+import io
 import json
 import os
 import sys
+import urllib.error
 import urllib.parse
 
 import pytest
@@ -128,7 +130,10 @@ class FakeLidarr:
     exercise the same stitching the real thing does.
     """
 
-    def __init__(self, artists=()):
+    def __init__(self, artists=(), fail=()):
+        # (path, artistId) pairs the fake refuses to serve, standing in for the
+        # 500s Lidarr returns on data it cannot hydrate.
+        self.fail = set(fail)
         self.artists, self.albums, self.tracks, self.files = [], [], [], []
         for artist_index, entry in enumerate(artists, start=1):
             artist_id = artist_index
@@ -190,10 +195,26 @@ class FakeLidarr:
         }
         self.calls.append((path, query))
 
+        artist_id = int(query.get("artistId", 0))
+        if (path, artist_id) in self.fail:
+            raise urllib.error.HTTPError(
+                url, 500, "Internal Server Error", {}, io.BytesIO(b'{"message": "boom"}')
+            )
+
         if path == "artist":
             return json.dumps(self.artists)
-        artist_id = int(query.get("artistId", 0))
         source = {"album": self.albums, "track": self.tracks, "trackfile": self.files}[path]
+        if path == "album" and not artist_id:
+            # Lidarr's unfiltered album endpoint returns the lot.
+            return json.dumps(source)
+        if not artist_id:
+            raise urllib.error.HTTPError(
+                url,
+                400,
+                "Bad Request",
+                {},
+                io.BytesIO(b'{"message": "artistId must be provided"}'),
+            )
         return json.dumps([row for row in source if row["artistId"] == artist_id])