Protect a show's Tier 3 pending episode from retention reclaim

Peter's call (option 1 of the three in docs/detection-findings.md): exempt
an episode retention would otherwise reclaim while it is a show's pending
comparison basis, rather than extending the grace period or accepting the
gap.

RetentionSweeper gains a pendingEpisodeIds seam (suspend () -> Set<String>,
defaulting to empty) and filters its reclaim candidates against it before
the grace-period policy ever sees them -- every existing caller and test
was unaffected by the default. RetentionWorker wires the real thing: the
union of two new, narrow DAO queries --
ShowFingerprintDao.getAllPendingEpisodeIds() (intro/outro) and
MidrollDao.getAllMidrollPendingEpisodeIds() (midroll, reading
SubscriptionEntity.midrollPendingEpisodeId).

Applied to both fingerprinting mechanisms, not just the one that got
diagnosed: MidrollUpdater's discovery has the identical rolling-pending-
episode shape and would race retention the same way for a fast show, just
silently, with no frozen row to reveal it.

Not yet re-verified on a device -- the next live test is two Up First
downloads close enough together to actually compare, confirming
pendingEpisodeId advances to a real second episode instead of restarting.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Sj3CoWEedM56bebByKgNxA

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