internal/service: log and concurrently process podcast feeds (task 91)

- Add *slog.Logger to podcastService with NewPodcastServiceWithLogger.
- Wire logger from cmd/player main.go.
- Run CheckFeeds concurrently: one goroutine per feed with WaitGroup.
- Add structured logging for start/finish per feed and overall count.
- Log feed failures at Warn level so they are visible but not fatal.
- Ensure nil logger safety with slog.Default fallback.
- Add tests: empty list, concurrent OK, list error, single feed error,
  all feeds error.

0 files changed, 0 insertions, 0 deletions