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

1 files changed, 3 insertions, 0 deletions

diff --git a/internal/api/handlers_test.go b/internal/api/handlers_test.go
index fc01ac0..eaeb665 100644
--- a/internal/api/handlers_test.go
+++ b/internal/api/handlers_test.go
@@ -1536,6 +1536,9 @@ func (m *mockPingStore) GetEpisodeByGUID(ctx context.Context, feedID int64, guid
 func (m *mockPingStore) ListEpisodesByFeed(ctx context.Context, feedID int64, limit, offset int) ([]model.PodcastEpisode, error) {
 	return m.store.ListEpisodesByFeed(ctx, feedID, limit, offset)
 }
+func (m *mockPingStore) ListEpisodesByFeedIDsWithStatus(ctx context.Context, userID int64, feedIDs []int64, limit, offset int) ([]model.PodcastEpisodeWithStatus, error) {
+	return m.store.ListEpisodesByFeedIDsWithStatus(ctx, userID, feedIDs, limit, offset)
+}
 func (m *mockPingStore) UpdateEpisodeMedia(ctx context.Context, episodeID, mediaID int64, fileName string) error {
 	return m.store.UpdateEpisodeMedia(ctx, episodeID, mediaID, fileName)
 }