Fix AV1 validation false positives for "Corrupt frame detected" errors

[Copilot]

AV1 files were incorrectly rejected during transcode validation when ffmpeg reported "Corrupt frame detected" or "No sequence header" errors. These are known false positives for AV1 files with temporal scalability or non-standard sequence header placement—files play correctly but fail validation.

Changes

• Restructured AV1 validation logic to check for false positives regardless of ffmpeg return code (previously only handled when returncode == 0)
• Track known false positives separately from real corruption indicators
• Pass validation if only false positives are found, even with non-zero exit code
• Fail appropriately when returncode is non-zero with empty stderr

Behavior

Scenario	Before	After
AV1 file + "Corrupt frame detected" + returncode=1	❌ Rejected	✅ Passes
AV1 file + "moov atom not found" + returncode=1	❌ Rejected	❌ Rejected
Non-AV1 file + "Corrupt frame detected"	❌ Rejected	❌ Rejected

Known AV1 false positive indicators:

• Corrupt frame detected
• No sequence header
• Error submitting packet to decoder
• Decode error rate
• Invalid NAL unit size
• non-existing PPS

Original prompt

When trying to transcode some AV1 encoded files I get this error. The files play just fine so I know they are not corrupted. Is there some way we can better detect a working av1 file and attempt to transcode it than what is already being done?

2026-01-19 18:48:12,844 INFO cli.transcode_videos:569 | Transcoding 034fa386784a4b15c0d47bfa3446fc17 to 720p
2026-01-19 18:48:13,384 ERROR util.transcode_video_quality:503 | Source video validation failed: Video file appears to be corrupt: Corrupt frame detected

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