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

0 files changed, 0 insertions, 0 deletions