Count only subscribers' read marks and pins when freeing disk space (#156)

The reaper counted read marks and pins in entry_state from anyone and compared them with the feed's
subscribers. Unsubscribing leaves your rows behind, and the flags endpoint writes them for feeds you
do not subscribe to as well. So an episode read by someone who had left and by one subscriber of
two counted as heard by everyone, and went first under the quota though the other had not played
it; and a pin of a person who had left kept a file, and its item, for good, which can leave the
quota impossible to meet.

Both queries, the reaper's candidates and prune_entries, now count only the marks of people who
subscribe to the item's feed. The old prune test's pin had no subscriber behind it; it has one now.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Freeing disk space counts only the people who subscribe to a feed: an episode is no longer taken as heard by everyone because someone who has since unsubscribed heard it, and their pins no longer keep files for good.
 - An episode whose file turns out to be a torrent shows as downloaded once it is, instead of being fetched again on every check of its feed, and no longer holds up the checking of other feeds while it downloads and seeds.
 - Downloading the latest 65 to 100 items of a feed queues them at once, instead of hanging until most had downloaded and holding up adding feeds and Refresh meanwhile.
 - Changes to the feed list made at the same moment, such as two people adding feeds, or a feed added while the server settings change, no longer undo one another.