Refresh skips on feed refresh, without stamping server offsets onto publisher audio

A feed refresh never re-asked the skip server for already-downloaded episodes, so
detection improvements only ever reached a show by deleting and redownloading it.
Fixing that safely needs to know which episodes are safe to re-query: a show using
dynamic ad insertion serves a different file on every request, so a server-measured
offset only applies to the server's own copy, not whatever the publisher fallback
left on disk.

- EpisodeEntity.audioFromServer (schema 13->14, MIGRATION_13_14) records which source
  actually landed; EpisodeDownloader sets it, defaulting to false everywhere else so a
  requeue, retention reclaim or failed re-download clears a stale claim the same way
  they already clear localPath.
- New SkipRefresher re-queries downloaded, server-sourced episodes via the existing
  ServerSkips.resolve/record pair, one episode's failure not blocking the rest. Wired
  into SubscriptionRepository.refreshAll via a new refreshSkips collaborator -- the
  choke point every refresh route passes through, including the library's refresh
  button.
- SkipSummary's episode screen readout gets a distinct line for a download that can
  never be refreshed in place, instead of the misleading "Not checked yet".

Migration verified on-device (gradle-verify connectedDebugAndroidTest, MigrationTest:
14/14). Full connected suite: 31/31. Unit suite: 682/682. detekt/lint clean; coverage
93.86% against a 93.85% baseline.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

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