Remove DeadAir copies unplayed for a week, and remake them on play (#179)

A copy is nearly the size of its original and was kept as long as the
original. Now `deadair.played_at` notes when one was last served (at
most once an hour, since a player asks again at every seek), and each
pass of the maker removes copies unplayed, or unmade, for 7 days. Only
the file goes: the row keeps its cuts with its path cleared, which
tells an expired copy apart from a file that cannot have one (no cuts)
and keeps the background queue from making it again straight away.
Serving the original of such an episode deletes the row, which puts it
back on the queue; the copy then comes back within a pass, though the
page does not switch to it mid-play (#178).

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

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - The container is built on Debian 13 (trixie) instead of 12, so DeadAir uses ffmpeg 7.1 instead of 5.1 (#177).
+- DeadAir removes a shortened copy nobody has played for a week, to save space, and makes it again when someone next plays the episode (#179).
 
 ## [0.11.0] - 2026-10-09