w0: Split SubscribeFeed into smaller helpers

SubscribeFeed was ~100 lines and did too much: admin check, feed parsing,
filesystem setup, DB set creation, permission grant, feed creation, cover
download, episode insertion, and marking checked.

Refactored into focused helpers:
- verifyAdmin          – admin gate
- resolveSetPath       – sanitize name and build disk path
- createPodcastSet     – mkdir, create set row, grant owner perm
- rollbackSet          – delete set row + remove directory
- createPodcastFeed    – insert feed row
- insertPodcastEpisodes – bulk insert parsed episodes

Injected dependency fields (parseFeed, parseFeedReader, downloadCover)
into podcastService so unit tests can replace them with fakes instead of
hitting real HTTP or disk.

Added internal/service/podcast_test.go with coverage for:
- happy path SubscribeFeed
- non-admin rejection
- user lookup error
- feed parse error
- set creation error (directory cleaned up)
- grant permission error (rollback)
- feed creation error (rollback)
- resolveSetPath / sanitize helpers
- DownloadEpisode permission check
- upsertFeedEpisodes (new + skip existing)
- updateFeedFromParsed
- insertPodcastEpisodes

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