[nightly-review] Prune old failed meeting audio

[r3dbars]

Summary

• Wires the existing FailedTranscriptionManager age cleanup into MeetingSessionController startup.
• Adds a named 30-day failed-meeting audio retention constant.
• Adds fast regression coverage so the cleanup hook does not disappear again.

Nightly evidence

• Reviewed PRs [nightly-regression] Cover live capture smoke command contract #800-Fix release gate blockers #817 and current origin/main.
• Live Sentry still shows one 1.1.42 CoreML/objc_release fatal before the main-branch lifetime fix landed; next release should carry that fix.
• PR Fix release gate blockers #817 intentionally preserved failed recordings, but the old cleanup method had zero call sites, so stale failed audio could accumulate indefinitely.

Verification

• bash build-deps.sh --force
• bash build.sh --no-open
• bash run-tests.sh
• bash run-integration-smoke.sh

[r3dbars]

verdict: no-findings-but-human

findings: none from this pass.
actions: human signoff recommended because this is draft behavior that deletes old failed-meeting audio after the 30-day retention window.
head_sha: 4323652
fix_commit: none
checks: gh pr view, gh pr diff, Meeting/Support/Test docs read, FailedTranscriptionManager cleanup path reviewed, git diff --check origin/main...origin/codex/nightly-review-failed-meeting-cleanup-20260520; GitHub checks reported none.

manual_merge_handoff:
head_sha: 4323652
verdict: no-findings-but-human
blockers: draft PR plus destructive failed-audio retention behavior needs human signoff
checks: gh metadata/diff, local docs/static review, cleanup path review, diff check