fix(recording): prevent orphaned rotation processes

[Serph91P]

Summary

• Serializes recording rotation per stream and preserves process ownership until the previous subprocess has exited.
• Uses bounded terminate and wait, followed by kill and wait only after a timeout.
• Prevents a stale segmented-process monitor from finalizing a recording after rotation replaced its owner.
• Adds deterministic local fake-process coverage for rotation lifecycle, failure, cancellation, stale ownership, concurrent rotation behavior, and immediate-exit ownership cleanup.
• Wraps the three rotation log messages to comply with the 88-character Python line limit without changing their meaning.

Verification

• Exact immutable head: 25bdfd5aeab3d37eac48da15717176d476a1c3fd on fix/781-rotation-orphans, based on develop at 326c77b8cad3fc35881ff2d3d21b8f31656c2e07.
• Focused rotation and recovery suite passed: 16 passed.
• Full backend suite passed: 218 passed, 2 skipped.
• Ruff check and format check passed for the touched recording source and regression tests.
• Migration initialization smoke check passed.
• git diff --check and the forbidden Unicode dash scan passed.
• All 17 current-head GitHub checks passed, including Docker Build Test.
• Local Docker Buildx is unavailable, so the linux/amd64 image build was verified by the exact-head CI gate.

Closes #781