feat(server/m7-352): kind filter on GET /api/playlists + lazy fallback

Adds ?kind= filter (user|system|all, default user) to GET /api/playlists
so system mixes don't leak into slice-1 SPA views. Lazy background build
fires when kind=system|all and the caller's run row is stale (>24h).
Propagates Kind/SystemVariant/SeedArtistID through PlaylistRow and
playlistRowView wire types.

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

internal/api/playlists_test.go

@@ -454,3 +454,118 @@ func TestPlaylists_CoverNoneReturns404(t *testing.T) {
 		t.Errorf("status = %d, want 404; body = %s", w.Code, w.Body.String())
 	}
 }
+
+// TestListPlaylists_KindFilterDefaultsToUser verifies that an absent ?kind=
+// parameter defaults to "user" and excludes system-kind playlists.
+func TestListPlaylists_KindFilterDefaultsToUser(t *testing.T) {
+	h, pool := testHandlers(t)
+	truncateLibrary(t, pool)
+	u := seedUser(t, pool, "klistdefault", "pw", false)
+
+	// Create one user-kind playlist via the service.
+	userPL, err := h.playlists.Create(context.Background(), u.ID, "My PL", "", false)
+	if err != nil {
+		t.Fatalf("create user playlist: %v", err)
+	}
+	// Insert a system-kind playlist directly via SQL.
+	if _, err := pool.Exec(context.Background(), `
+		INSERT INTO playlists (user_id, name, description, is_public, kind, system_variant)
+		VALUES ($1, 'For You', '', false, 'system', 'for_you')
+	`, u.ID); err != nil {
+		t.Fatalf("seed system playlist: %v", err)
+	}
+
+	w := doPlaylistsReq(h, u, "GET", "/api/playlists", nil)
+	if w.Code != http.StatusOK {
+		t.Fatalf("status: got %d, want 200; body=%s", w.Code, w.Body.String())
+	}
+	var resp listPlaylistsResponse
+	if err := json.Unmarshal(w.Body.Bytes(), &resp); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(resp.Owned) != 1 {
+		t.Fatalf("default should return 1 user-kind playlist; got %d", len(resp.Owned))
+	}
+	if resp.Owned[0].Kind != "user" {
+		t.Errorf("expected kind=user; got %q", resp.Owned[0].Kind)
+	}
+	if resp.Owned[0].ID != uuidToString(userPL.ID) {
+		t.Errorf("returned wrong playlist id")
+	}
+}
+
+// TestListPlaylists_KindSystem verifies ?kind=system returns only
+// system-generated playlists and excludes user-created ones.
+func TestListPlaylists_KindSystem(t *testing.T) {
+	h, pool := testHandlers(t)
+	truncateLibrary(t, pool)
+	u := seedUser(t, pool, "klistsys", "pw", false)
+
+	if _, err := h.playlists.Create(context.Background(), u.ID, "User PL", "", false); err != nil {
+		t.Fatalf("create user playlist: %v", err)
+	}
+	if _, err := pool.Exec(context.Background(), `
+		INSERT INTO playlists (user_id, name, description, is_public, kind, system_variant)
+		VALUES ($1, 'For You', '', false, 'system', 'for_you')
+	`, u.ID); err != nil {
+		t.Fatalf("seed system playlist: %v", err)
+	}
+
+	w := doPlaylistsReq(h, u, "GET", "/api/playlists?kind=system", nil)
+	if w.Code != http.StatusOK {
+		t.Fatalf("status: got %d, want 200; body=%s", w.Code, w.Body.String())
+	}
+	var resp listPlaylistsResponse
+	if err := json.Unmarshal(w.Body.Bytes(), &resp); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(resp.Owned) != 1 {
+		t.Fatalf("kind=system should return 1 system playlist; got %d", len(resp.Owned))
+	}
+	if resp.Owned[0].Kind != "system" {
+		t.Errorf("expected kind=system; got %q", resp.Owned[0].Kind)
+	}
+}
+
+// TestListPlaylists_KindAll verifies ?kind=all returns all playlists
+// regardless of kind.
+func TestListPlaylists_KindAll(t *testing.T) {
+	h, pool := testHandlers(t)
+	truncateLibrary(t, pool)
+	u := seedUser(t, pool, "klistall", "pw", false)
+
+	if _, err := h.playlists.Create(context.Background(), u.ID, "User PL", "", false); err != nil {
+		t.Fatalf("create user playlist: %v", err)
+	}
+	if _, err := pool.Exec(context.Background(), `
+		INSERT INTO playlists (user_id, name, description, is_public, kind, system_variant)
+		VALUES ($1, 'For You', '', false, 'system', 'for_you')
+	`, u.ID); err != nil {
+		t.Fatalf("seed system playlist: %v", err)
+	}
+
+	w := doPlaylistsReq(h, u, "GET", "/api/playlists?kind=all", nil)
+	if w.Code != http.StatusOK {
+		t.Fatalf("status: got %d, want 200; body=%s", w.Code, w.Body.String())
+	}
+	var resp listPlaylistsResponse
+	if err := json.Unmarshal(w.Body.Bytes(), &resp); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(resp.Owned) != 2 {
+		t.Fatalf("kind=all should return both playlists; got %d", len(resp.Owned))
+	}
+}
+
+// TestListPlaylists_BadKindReturns400 verifies an unrecognised kind value
+// is rejected with 400 bad_kind.
+func TestListPlaylists_BadKindReturns400(t *testing.T) {
+	h, pool := testHandlers(t)
+	u := seedUser(t, pool, "klistbad", "pw", false)
+	_ = pool
+
+	w := doPlaylistsReq(h, u, "GET", "/api/playlists?kind=garbage", nil)
+	if w.Code != http.StatusBadRequest {
+		t.Fatalf("expected 400; got %d, body=%s", w.Code, w.Body.String())
+	}
+}