feat: AssemblyAI webhook migration (stage 0 dark deploy)

[dtrn2048]

Summary

• implement AssemblyAI webhook-capable transcription flow for Dembrane-25-09
• keep polling behavior unchanged when webhook env is unset (dark deploy path)
• add webhook endpoint, coordination primitives, settings validation, and tests

Validation\n- uv run ruff check dembrane tests

• uv run pytest tests/test_transcribe_webhook.py tests/test_tasks_webhook.py tests/api/test_webhooks.py tests/test_settings_webhook.py -q
• uv run pytest tests/ -v -m "not integration and not slow and not smoke" --maxfail=3

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/eventdriventranscription

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