fix(async-audit): Phase 3 cleanup — eliminate sync I/O in async routes

[pcalnon]

Summary

Phase 3 of the async-route audit migration (see juniper-ml ASYNC_ROUTE_AUDIT_HOOK_MIGRATION_PLAN.md). Clears the remaining 5 ASYNC* violations in this repo so we can flip the hook from soft-fail to enforcement in Phase 4.

After this lands, ruff check --select ASYNC src/ reports All checks passed!

Changes

src/main.py

• ASYNC240 — Snapshot detail route (GET /api/v1/snapshots/{snapshot_id}) now calls a new _find_snapshot_file helper via asyncio.to_thread. Previously did path.exists() + path.iterdir() + snapshot_file.stat() inline (three sync stat-bound calls on the event loop); now bundled into a single thread hop.
• ASYNC240 — Snapshot create route (POST /api/v1/snapshots) wraps Path(_snapshots_dir).mkdir(parents=True, exist_ok=True) in asyncio.to_thread so a slow disk can't stall the loop.
• ASYNC109 — api_remote_stop_workers's timeout: int = 10 parameter is a passthrough to backend._adapter.stop_remote_workers, not a deadline for this handler. Silenced with # noqa: ASYNC109 and a justification comment.

src/backend/cascor_service_adapter.py

• ASYNC110 — Control-stream supervisor's while not self._shutdown and self.is_connected: await asyncio.sleep(1) keeps its 1s poll cadence with # noqa: ASYNC110. Rewriting around asyncio.Event would require touching the stream callback chain we don't own, and 1s latency is well below any reconnect we care about.

New helpers in src/main.py (added in a prior commit on this branch):

• _load_snapshot_history(history_file) — reads snapshot_history.jsonl; tolerates missing files and malformed lines.
• _find_snapshot_file(snapshots_dir, snapshot_id) — bundles exists + iterdir + stat for callers using asyncio.to_thread.

Verification

• ruff check --select ASYNC src/ exits clean
• pre-commit suite green on both files (black, isort, flake8, mypy, bandit, async-audit)
• Helper smoke test: missing-file → [], malformed JSON tolerated, snapshot lookup hit/miss/missing-dir all behave correctly
• CI pre-commit + tests
• Local pytest currently blocked by an unrelated torch ImportError in the JuniperCanopy 3.14t conda env (see juniper-ml memory: "JuniperCascor torch ImportError"); not introduced by this PR

Test plan

• Pre-commit + flake8 + mypy + bandit pass in CI
• Async-audit job is now an unconditional pass (Phase 2 was soft-fail; Phase 4 will flip --exit-zero and continue-on-error off after this lands)
• Unit suite green in CI environment with working torch

🤖 Generated with Claude Code

[pcalnon]

approved