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/client_test.go

@@ -154,6 +154,12 @@ func TestLookupAlbum_HappyPath(t *testing.T) {
 	if got[0].MBID != "a1b2c3d4-e5f6-7890-abcd-ef1234567890" {
 		t.Errorf("MBID = %q", got[0].MBID)
 	}
+	if got[0].ArtistMBID != "069b64b6-7884-4f6a-94cc-e4c1d6c87a01" {
+		t.Errorf("ArtistMBID = %q, want 069b64b6-7884-4f6a-94cc-e4c1d6c87a01", got[0].ArtistMBID)
+	}
+	if got[0].AlbumMBID != "" {
+		t.Errorf("AlbumMBID = %q, want empty on album result", got[0].AlbumMBID)
+	}
 	if got[0].Name != "Music Has the Right to Children" {
 		t.Errorf("Name = %q", got[0].Name)
 	}
@@ -213,6 +219,12 @@ func TestLookupTrack_HappyPath(t *testing.T) {
 	if got[0].Secondary != want {
 		t.Errorf("Secondary = %q, want %q", got[0].Secondary, want)
 	}
+	if got[0].ArtistMBID != "069b64b6-7884-4f6a-94cc-e4c1d6c87a01" {
+		t.Errorf("ArtistMBID = %q, want 069b64b6-7884-4f6a-94cc-e4c1d6c87a01", got[0].ArtistMBID)
+	}
+	if got[0].AlbumMBID != "a1b2c3d4-e5f6-7890-abcd-ef1234567890" {
+		t.Errorf("AlbumMBID = %q, want a1b2c3d4-e5f6-7890-abcd-ef1234567890", got[0].AlbumMBID)
+	}
 	if got[0].ImageURL != "" {
 		t.Errorf("tracks carry no images; ImageURL = %q", got[0].ImageURL)
 	}
@@ -406,6 +418,51 @@ func TestListQualityProfiles_BadJSON(t *testing.T) {
 	}
 }
 
+// --- LookupArtist (additional) ---
+
+func TestLookupArtist_LookupFailed(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+	}))
+	defer srv.Close()
+	c := &Client{BaseURL: srv.URL, APIKey: "x", HTTP: srv.Client()}
+	_, err := c.LookupArtist(context.Background(), "boards")
+	if !errors.Is(err, ErrLookupFailed) {
+		t.Fatalf("err = %v, want ErrLookupFailed", err)
+	}
+}
+
+func TestGet_BaseURLWithTrailingSlash(t *testing.T) {
+	body, _ := os.ReadFile("testdata/lookup_artist.json")
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// Verify path is /api/v1/artist/lookup, NOT //api/v1/artist/lookup
+		if r.URL.Path != "/api/v1/artist/lookup" {
+			t.Errorf("got path %q; trailing-slash BaseURL produced double-slash", r.URL.Path)
+		}
+		_, _ = w.Write(body)
+	}))
+	defer srv.Close()
+	c := &Client{BaseURL: srv.URL + "/", APIKey: "k", HTTP: srv.Client()}
+	_, err := c.LookupArtist(context.Background(), "boards")
+	if err != nil {
+		t.Fatalf("LookupArtist: %v", err)
+	}
+}
+
+// --- AddArtist (additional) ---
+
+func TestAddArtist_PostFailed(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusUnprocessableEntity)
+	}))
+	defer srv.Close()
+	c := &Client{BaseURL: srv.URL, APIKey: "x", HTTP: srv.Client()}
+	err := c.AddArtist(context.Background(), AddArtistParams{ForeignArtistID: "x", QualityProfileID: 1, RootFolderPath: "/m"})
+	if !errors.Is(err, ErrLookupFailed) {
+		t.Fatalf("err = %v, want ErrLookupFailed", err)
+	}
+}
+
 // --- ListRootFolders ---
 
 func TestListRootFolders_HappyPath(t *testing.T) {
@@ -439,7 +496,7 @@ func TestListRootFolders_HappyPath(t *testing.T) {
 	}
 }
 
-func TestListRootFolders_BadJSON(t *testing.T) {
+func TestListRootFolders_NullBody(t *testing.T) {
 	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
 		_, _ = w.Write([]byte(`null`))
 	})
@@ -454,6 +511,18 @@ func TestListRootFolders_BadJSON(t *testing.T) {
 	}
 }
 
+func TestListRootFolders_BadJSON(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		_, _ = w.Write([]byte("{not valid json"))
+	}))
+	defer srv.Close()
+	c := &Client{BaseURL: srv.URL, APIKey: "x", HTTP: srv.Client()}
+	_, err := c.ListRootFolders(context.Background())
+	if !errors.Is(err, ErrInvalidPayload) {
+		t.Fatalf("err = %v, want ErrInvalidPayload", err)
+	}
+}
+
 // --- Ping ---
 
 func TestPing_ReturnsVersion(t *testing.T) {
@@ -507,11 +576,7 @@ func TestPing_BadJSON(t *testing.T) {
 // --- pickPosterImage ---
 
 func TestPickPosterImage_PreferRemoteURL(t *testing.T) {
-	imgs := []struct {
-		CoverType string `json:"coverType"`
-		RemoteURL string `json:"remoteUrl"`
-		URL       string `json:"url"`
-	}{
+	imgs := []lidarrImage{
 		{CoverType: "poster", RemoteURL: "https://remote.example/img.jpg", URL: "https://local.example/img.jpg"},
 	}
 	got := pickPosterImage(imgs)
@@ -521,11 +586,7 @@ func TestPickPosterImage_PreferRemoteURL(t *testing.T) {
 }
 
 func TestPickPosterImage_FallsBackToLocalURL(t *testing.T) {
-	imgs := []struct {
-		CoverType string `json:"coverType"`
-		RemoteURL string `json:"remoteUrl"`
-		URL       string `json:"url"`
-	}{
+	imgs := []lidarrImage{
 		{CoverType: "poster", RemoteURL: "", URL: "https://local.example/img.jpg"},
 	}
 	got := pickPosterImage(imgs)
@@ -535,11 +596,7 @@ func TestPickPosterImage_FallsBackToLocalURL(t *testing.T) {
 }
 
 func TestPickPosterImage_SkipsNonPoster(t *testing.T) {
-	imgs := []struct {
-		CoverType string `json:"coverType"`
-		RemoteURL string `json:"remoteUrl"`
-		URL       string `json:"url"`
-	}{
+	imgs := []lidarrImage{
 		{CoverType: "banner", RemoteURL: "https://banner.example/img.jpg"},
 		{CoverType: "cover", RemoteURL: "https://cover.example/img.jpg"},
 	}