Raise the ad-detector's noise floor from 4s to 12s, bridge pods at 60s

Owner reported a 38-minute episode with 37 breaks, 14 of them exactly
4.0 seconds and scattered roughly one per minute through the whole
thing: "This is madness. Ads are usually multiples of 15 seconds,
grouped together in a few spots." He was right. DefaultRunOptions's
MinRunSamples: 1000 is exactly 4.0s at the 250Hz discovery rate, so
anything that long was taken seriously -- that floor, not a real ad,
is why 14 breaks landed on precisely 4.0s: correlation noise pinned
at the floor.

Measured across 333 mirrored episodes: episodes the feed proves carry
advertising (served >=60s longer than declared) have a median of 3
breaks, max 7. All six badly-fragmented episodes in the corpus belong
to one show (Sleepy History), which reuses long ambient music beds --
the detector was matching the programme's own texture against itself.

Fix, validated against the declared-vs-served length gap (a measure
the detector does not produce and so can't flatter itself with):

- MinRunSamples 1000 -> 3000 (4s -> 12s). 12 rather than 15 so a real
  15s ad whose correlation lapses at one end still survives; both
  floors were measured and 12 scored better.
- AdConfig.MergeGapMs 10_000 -> 60_000, so fragments of one real ad
  pod separated by more than 10s bridge into the single break a
  listener experiences, rather than staying fragmented.

Corpus-wide: 1091 -> 539 breaks, agreement with the length gap 31% ->
34%, unverifiable skipping on no-insertion shows 73min -> 64min. Half
the breaks removed and accuracy up is the signal they were noise.

Both constants' doc comments rewritten to state the new value and why,
not just the mechanics -- the comments are the documentation here.

TDD: added TestDefaultRunOptionsFloorIsTwelveSeconds (no pin existed
for MinRunSamples) and updated TestDefaultAdConfigUsesTheValidatedValues's
MergeGapMs pin; both confirmed red against the old constants before the
change. Three existing ads_test.go fixtures built at or under 10-12s
were lengthened to clear the new floor (not weakened). Added
TestScatteredSubFloorFragmentsProduceNoBreaksButARealPodStillDoes,
confirmed it fails against the old constants and passes against the
new ones.

gofmt, go vet, go test ./... all clean (server/ only -- no app/ changes).

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

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