fix(lidarr): address review findings on HTTP client

- Add ArtistMBID/AlbumMBID to LookupResult so handlers can build the
  full lidarr_requests row from album/track lookups (parent MBID is
  needed for the AddAlbum API call and for the schema's NOT NULL
  lidarr_artist_mbid)
- Trim trailing slash on BaseURL before joining paths to avoid
  double-slash URLs when operators enter "http://lidarr.lan/"
- Check json.Marshal errors in AddArtist/AddAlbum
- Extract lidarrImage as a named unexported type instead of repeating
  the anonymous struct three times
- Add NewClient(baseURL, apiKey) constructor with 30s default timeout
- Add ErrLookupFailed test coverage for the 4xx-non-auth branch on
  both get and post helpers
- Rename TestListRootFolders_BadJSON -> _NullBody (it tests null,
  not malformed); add a real malformed-JSON test
- Defensive separator guard on LookupAlbum.Secondary so an empty
  ArtistName doesn't produce a leading " · "

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/lidarr/types.go

@@ -4,13 +4,25 @@
 package lidarr
 
 // LookupResult is the normalized shape returned by Lookup{Artist,Album,Track}.
-// It is what we store on the request row (via the user's request) and
-// what /api/lidarr/search returns to the SPA.
+// MBID is the kind's primary identifier (foreignArtistId / foreignAlbumId /
+// foreignTrackId from Lidarr). ArtistMBID and AlbumMBID carry parent
+// identifiers when the kind has them — request creation and admin
+// approval need them to construct the lidarr_requests row and the
+// AddArtist/AddAlbum API calls.
 type LookupResult struct {
-	MBID      string // foreignArtistId / foreignAlbumId / foreignTrackId
-	Name      string // artist name; album/track returns Title here too
-	Secondary string // genre + album count for artist; year for album; album for track
-	ImageURL  string // cover-art URL Lidarr surfaced (may be empty)
+	MBID       string // foreignArtistId / foreignAlbumId / foreignTrackId
+	ArtistMBID string // parent artist's foreignArtistId — set on album/track results, empty on artist results
+	AlbumMBID  string // parent album's foreignAlbumId — set on track results only
+	Name       string // artist name; album/track returns Title here too
+	Secondary  string // genre + album count for artist; year for album; album for track
+	ImageURL   string // cover-art URL Lidarr surfaced (may be empty)
+}
+
+// lidarrImage is the wire shape for image entries in Lidarr's API responses.
+type lidarrImage struct {
+	CoverType string `json:"coverType"`
+	RemoteURL string `json:"remoteUrl"`
+	URL       string `json:"url"`
 }
 
 // QualityProfile is the dropdown choice in /admin/integrations.