Add circuit breaker / backoff for failing podcast feeds in CheckFeeds

- DB: add consecutive_failures and next_check_at to podcast_feeds
- model: add ConsecutiveFailures and NextCheckAt to PodcastFeed struct
- repository: update CRUD queries to handle new columns and
  ListFeedsNeedingCheck to skip feeds with future next_check_at
- service: update CheckFeeds to use new query, update checkFeed to
  reset counters on success and increment exponential backoff on failure
- tests: adapt mock signatures, add backoff integration test

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