internal/service: fix ListEpisodes global pagination by using cross-feed SQL query

Previously ListEpisodes requested per-feed episodes with the full limit
from every feed, concatenated them, and then sliced the resulting slice
in memory. This silently skipped episodes from deeper pages because the
per-feed DB limit never returned them.

Fix: Add a new repository method ListEpisodesByFeedIDsWithStatus that
takes a slice of feed IDs and applies LIMIT/OFFSET globally in a single
SQL query (IN (...)). The service builds the feed ID list and delegates
pagination to the database instead of emulating it in memory.

Files changed:
- internal/repository/podcast.go: add ListEpisodesByFeedIDsWithStatus
- internal/repository/podcast_repo.go: extend PodcastRepo interface
- internal/repository/mock.go: add mock implementation
- internal/service/podcast.go: replace per-feed loop with new method
- internal/service/podcast_test.go: add tests for ListEpisodes
- internal/repository/podcast_test.go: add integration tests for new repo method
- internal/api/handlers_test.go: add mockPingStore method

2 files changed, 80 insertions, 15 deletions

diff --git a/internal/service/podcast.go b/internal/service/podcast.go
index 9c9e90d..1d4f16b 100644
--- a/internal/service/podcast.go
+++ b/internal/service/podcast.go
@@ -420,22 +420,11 @@ func (s *podcastService) ListEpisodes(ctx context.Context, setID, userID int64,
 		return nil, ErrNotFound
 	}
 
-	all := make([]model.PodcastEpisodeWithStatus, 0)
-	for _, feed := range feeds {
-		episodes, err := s.store.ListEpisodesWithStatus(ctx, userID, feed.ID, limit, 0)
-		if err != nil {
-			return nil, err
-		}
-		all = append(all, episodes...)
-	}
-	if offset >= len(all) {
-		return []model.PodcastEpisodeWithStatus{}, nil
-	}
-	end := len(all)
-	if limit > 0 && offset+limit < end {
-		end = offset + limit
+	feedIDs := make([]int64, len(feeds))
+	for i, f := range feeds {
+		feedIDs[i] = f.ID
 	}
-	return all[offset:end], nil
+	return s.store.ListEpisodesByFeedIDsWithStatus(ctx, userID, feedIDs, limit, offset)
 }
 
 func (s *podcastService) DownloadEpisode(ctx context.Context, episodeID, userID int64) (*model.Media, error) {
diff --git a/internal/service/podcast_test.go b/internal/service/podcast_test.go
index 3fb94f9..f630ee0 100644
--- a/internal/service/podcast_test.go
+++ b/internal/service/podcast_test.go
@@ -914,3 +914,79 @@ func TestPodcastService_CheckFeeds_FeedError_Continues(t *testing.T) {
 	}
 	mu.Unlock()
 }
+
+func TestPodcastService_ListEpisodes_GlobalPagination(t *testing.T) {
+	ctx := context.Background()
+	svc, store := setupPodcastService(t)
+
+	store.SetPermissionRepo = repository.MockSetPermissionRepo{
+		GetPermissionFunc: func(ctx context.Context, setID, userID int64) (*model.SetPermission, error) {
+			return nil, nil
+		},
+	}
+	store.UserRepo = repository.MockUserRepo{
+		GetUserByIDFunc: func(ctx context.Context, id int64) (*model.User, error) {
+			return &model.User{ID: id, IsAdmin: true}, nil
+		},
+	}
+
+	store.PodcastRepo = repository.MockPodcastRepo{
+		ListFeedsBySetIDFunc: func(ctx context.Context, setID int64) ([]model.PodcastFeed, error) {
+			return []model.PodcastFeed{
+				{ID: 1, SetID: setID, Title: "Feed A"},
+				{ID: 2, SetID: setID, Title: "Feed B"},
+			}, nil
+		},
+		ListEpisodesByFeedIDsWithStatusFunc: func(ctx context.Context, userID int64, feedIDs []int64, limit, offset int) ([]model.PodcastEpisodeWithStatus, error) {
+			// Verify service passes both feed IDs and the original limit/offset.
+			if len(feedIDs) != 2 {
+				t.Errorf("expected 2 feedIDs, got %d", len(feedIDs))
+			}
+			if limit != 3 {
+				t.Errorf("expected limit 3, got %d", limit)
+			}
+			if offset != 5 {
+				t.Errorf("expected offset 5, got %d", offset)
+			}
+			return []model.PodcastEpisodeWithStatus{
+				{PodcastEpisode: model.PodcastEpisode{ID: 1, Title: "Ep 1"}},
+				{PodcastEpisode: model.PodcastEpisode{ID: 2, Title: "Ep 2"}},
+				{PodcastEpisode: model.PodcastEpisode{ID: 3, Title: "Ep 3"}},
+			}, nil
+		},
+	}
+
+	result, err := svc.ListEpisodes(ctx, 42, 1, 3, 5)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(result) != 3 {
+		t.Fatalf("expected 3 episodes, got %d", len(result))
+	}
+}
+
+func TestPodcastService_ListEpisodes_NoFeeds(t *testing.T) {
+	ctx := context.Background()
+	svc, store := setupPodcastService(t)
+
+	store.SetPermissionRepo = repository.MockSetPermissionRepo{
+		GetPermissionFunc: func(ctx context.Context, setID, userID int64) (*model.SetPermission, error) {
+			return nil, nil
+		},
+	}
+	store.UserRepo = repository.MockUserRepo{
+		GetUserByIDFunc: func(ctx context.Context, id int64) (*model.User, error) {
+			return &model.User{ID: id, IsAdmin: true}, nil
+		},
+	}
+	store.PodcastRepo = repository.MockPodcastRepo{
+		ListFeedsBySetIDFunc: func(ctx context.Context, setID int64) ([]model.PodcastFeed, error) {
+			return []model.PodcastFeed{}, nil
+		},
+	}
+
+	_, err := svc.ListEpisodes(ctx, 42, 1, 10, 0)
+	if !errors.Is(err, ErrNotFound) {
+		t.Fatalf("expected ErrNotFound, got %v", err)
+	}
+}