fix(token_usage): replace threading.Lock with asyncio.Lock in record()

[mvanhorn]

Summary

TokenUsageManager.record() is an async method that uses self._file_lock = threading.Lock() to serialize file I/O. threading.Lock is a blocking primitive - when acquired inside an async method on the event loop thread, it blocks the entire event loop until released. Under concurrent calls this produces the deadlock captured in the py-spy stack dump in #1834.

Fix: Replace threading.Lock() with asyncio.Lock() for _file_lock and change with self._file_lock: to async with self._file_lock:. This is the same pattern already used in app/runner/manager.py, app/crons/manager.py, and app/mcp/manager.py.

The class-level threading.Lock in get_instance() is kept unchanged since that classmethod is synchronous and never awaits.

Changes

• src/copaw/token_usage/manager.py: Add import asyncio, change _file_lock from threading.Lock() to asyncio.Lock(), change with to async with in record()

Test plan

• ruff check passes
• Concurrent record() calls from multiple coroutines no longer deadlock

Closes #1834

This contribution was developed with AI assistance (Claude Code + Codex).

[mvanhorn]

Thanks for the review.