Follow the data: detect the advertising, not the intro

The property that broke intro detection is the property that makes this work.
Eight episodes mirrored within one minute are maximally dependent, because the
ad server inserts whatever campaign is running into all of them identically.
For intro detection that destroys the independence the confirmation threshold
needs -- which is how it came to confirm an advertisement on 8 of 8 episodes.
For ad detection it is the signal.

And unlike intro detection it is checkable against a number the detector does
not produce: the feed declares the ad-free length, the file served is longer,
and the difference is the advertising.

**Measured on the live deployment, Radiolab's eight mirrored episodes: the scan
accounted for 53:29 of the 59:39 the length gap implies, with every individual
episode between 71% and 105%.** That is the first time detection in this
project has been checked against external ground truth and agreed with it.

The old sliding search could never have done this -- one full-length
correlation per window position, 199 seconds for a 240-second clip. BestLags
computes the cross-correlation over every lag in a single FFT pair, which says
where two recordings align; MatchedRuns then walks the extent of the agreement
directly, window by window, with no transform. Sized against measurements
rather than guesses: 2^22 is 1.69s, so a 70-minute episode decimated to 250Hz
is about five seconds a pair. Silence is explicitly not agreement, or separate
advertisements weld together across the pauses between them.

Breaks publish as ranges into episode_midrolls, which MidrollSkip and
PlayheadPolicy already read -- no playback changes and no migration. Intro and
outro offsets are still computed and logged but no longer published, and the
ad-derived ones already in the server's state were cleared.

Two guards publish nothing rather than something wrong: an episode whose
matches cover over 35% of it, and any single match over four minutes. A
re-aired episode or a two-parter would otherwise be eaten. Both record the
refusal, because "found nothing" and "refused what it found" are different
states and conflating them is the readout mistake over again.

Also three data-loss bugs, found by reading the journal after a restart rather
than by any test. The subscription list -- the one thing the server cannot
rediscover -- was persisted only when a whole poll cycle finished, which for 61
feeds is hours. A cycle re-applied its stale snapshot at the end, which would
undo a push that arrived during it and then delete the audio of every show the
snapshot omitted. And detection ignored cancellation, so systemd's 90-second
stop SIGKILLed the process before the save-on-exit could run. All three fixed.

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

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