Add fixture-backed transcription router media plan

[WalksWithASwagger]

Summary

• Add opt-in ffprobe media inspection and planned-only FFmpeg normalization metadata.
• Expose router output contracts plus privacy and cost receipts without changing runtime transcription defaults.
• Add fixture-backed coverage for cloud, local/private, chunked, timestamped, diarization-capable, video, and no-default-probe paths.

Verification

• PYTHON=/Users/kk/Code/spektorAI/venv/bin/python make lint
• PYTHON=/Users/kk/Code/spektorAI/venv/bin/python make test -> 288 passed
• PYTHON=/Users/kk/Code/spektorAI/venv/bin/python /Users/kk/Code/spektorAI/venv/bin/python -m pytest tests/test_audio.py -q -> 19 passed
• git diff --check

Closes #50

[github-actions]

Agentic PR Review

Verdict: needs-human

Checks:

• Linked issue via Closes #...: yes
• Issue acceptance criteria found: 4
• PR verification section present: yes
• PR acceptance self-check present: no
• PR self-check covers issue acceptance: no
• Diff within v1 limits: no
• Missing acceptance coverage:
  • Large audio/video inputs can be inspected and normalized through an FFmpeg-backed plan before transcription.
  • Router fixtures cover local/private, cloud, chunked, timestamped, and diarization-capable paths.
  • Runtime defaults remain unchanged until the router has explicit privacy and cost receipts.
  • Docs connect the provider matrix to the implemented router behavior.

This PR is not blocked from human review, but the agentic gate is not satisfied.