fix(windows): retry locked MP4 finalization

[petercr]

Summary

• retry transient Windows file-lock errors while syncing staged MP4 exports
• keep MP4 as the preferred completed artifact instead of immediately falling back to MKV recovery
• retain MKV fallback for non-transient export/finalization failures

Verification

• cargo fmt --all
• focused cargo test -p videorc-backend staged_mp4_sync_flushes_a_nonempty_file_and_rejects_an_empty_one was started but not completed in the local time window

Summary by CodeRabbit

• Bug Fixes
  • Improved Windows recording publication reliability by retrying transient file-lock failures after recording ends.
  • Non-transient synchronization errors continue to fail immediately.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 355d92de-6f21-4fda-8b64-fcd0d097305f

📥 Commits

Reviewing files that changed from the base of the PR and between 44264f9 and f533677.

📒 Files selected for processing (1)

• crates/videorc-backend/src/recording.rs

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📝 Walkthrough

Walkthrough

MP4 staging synchronization now retries transient Windows file-lock failures using bounded exponential backoff. Non-transient errors and exhausted retries continue to fail immediately.

Changes

MP4 staging synchronization

Layer / File(s)	Summary
Retry policy and lock detection crates/videorc-backend/src/recording.rs	Adds retry count and backoff constants, plus Windows-specific detection of transient file-lock errors across the error chain.
Bounded synchronization retry loop crates/videorc-backend/src/recording.rs	Replaces single-shot staging synchronization with a respawned retry loop that doubles backoff up to the configured maximum.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: theorcdev

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly captures the main change: Windows-specific retries for locked MP4 finalization.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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