fix(live): unblock long-GOP HLS and iPhone loading failures

[christopherklint97]

Summary

• Reproduced real NFL REPLAY INFO HD failure: valid 8.334/8.333-second GOP segments were rejected by a four-second publication guard, permanently disabling shared HLS and forcing MPEG-TS fallback.
• Admit finite, positive segments up to a fixed 12-second bound and advertise TARGETDURATION:12; preserve byte/storage/watchdog/strict packet-seam safety.
• Distinguish permanent unsafe rejection (immediate 501, no Retry-After) from genuine retryable 503 warmup; preserve latches and old-ticket invalidation. Log only channel ID and fixed retirement category.
• Keep Hls.js headroom at 24 seconds independent of advertised target and catch-up seeks disabled; iPhone/native selection remains intact.
• Show actionable errors for rejected legacy HTTP responses, destroy failed players, ignore late error/waiting callbacks, and preserve explicit retry. Initial zero-time updates no longer shorten first-frame acquisition.

Verification

• RED-to-GREEN real FFmpeg eight-second-GOP integration; policy, retirement and stale-ticket regressions.
• All frontend tests: 347 passed; typecheck/lint/build passed.
• Python worker/stitch: 24 passed.
• Focused backend tests and typecheck passed; full packaged CI required before merge/deployment.
• Initial host full-suite run used a 4-GiB tmpfs below the archive fixture's 20-GiB reserve and failed capacity checks; rerun on ordinary scratch filesystem in progress. Do not count that run as green.
• Real affected-channel candidate browser acceptance and independent review in progress.
• Physical iPhone playback not yet verified; local browser tests do not establish device acceptance.

Release

Exact-head CI, merge to main, verified ARM64 runtime deployment, and production readback/playback validation required before completion. No archive implementation changes.