DeadAir on the server: copies with long silences shortened (#171)

For anyone who turns DeadAir on, a background task makes each episode a
copy with every silence quieter than -38 dB for over 0.35 s shortened to
0.2 s. Nothing is re-encoded: ffmpeg copies the original's frames and
leaves the silent ones out, about 11 s for an hour of MP3 in a release
build. On Brad & Will's episode 359 that is 520 cuts and 3.3 minutes
saved, the copy decodes clean, and it has no isolated spikes in its
silences that the original lacks.

Cuts are made from the middle of each silence, keeping 0.1 s at each
end, so the frames either side of a cut are silent: decoding blends a
frame with the one before it, and an MP3 frame can begin in the bytes of
the frame before, so the first frame after a cut decodes against the
wrong neighbour. Inside a silence nobody hears that; at a word they would.

Positions stay in the original's time and the server converts them both
ways. The iOS and Android apps save positions themselves from their own
player, even while the page is asleep, so converting in the page would
have stored the copy's times as the original's. /media/{id}/deadair
serves the copy at its own address and records that this person is
playing it; a position saved meanwhile is converted from the copy's
clock, as is the length a player measures.

The copy is made by a task of its own, not the worker: a first pass over
a library is hours of ffmpeg, and scans must not wait behind it. It
checks every two minutes, which covers new downloads, files already
downloaded, a switch turned on and a file downloaded again with one rule.
Episodes everyone who would hear the copy has finished get none.
download::remove deletes a copy with its original, and the quota counts
copies.

Settings: the switch is PATCH /api/me {deadair}, a feed kept out is
PATCH /api/feeds/{id} {deadair: false}. GET /api/me gives the switch and
deadair_saved, the seconds saved on episodes heard 75% or more on a copy,
each counted once. Three new tables, so no ALTER.

The image gains Debian's ffmpeg, about 480 MB, in a layer that stays
cached until the base image changes. The player's switch, the feed
setting and the stat come next, after mockups.

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

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- DeadAir, on the server: for anyone who turns it on, each episode gets a copy with its long silences shortened, so podcasts finish sooner without sounding sped up, and your place carries over between the copy and the original. The switch in the player comes next (#171).
+
 ### Fixed
 
 - Cover art up to 20 MB is shown, instead of nothing for anything over 5 MB, as for five of the Glass Cannon Network's feeds.