[codex] fix(worker): cache missing schemas_with_pending_work helper

[JulienJBO]

Summary

• cache the absence of the optional schemas_with_pending_work() helper after the first UndefinedFunctionError
• keep the existing per-schema fallback path unchanged
• add a regression test that verifies the helper is only probed once when it is not installed

Root Cause

WorkerPoller._scan_active_schemas() tried SELECT * FROM schemas_with_pending_work() on every poll tick until success. In local and self-hosted deployments where that optional helper function is not installed, the worker correctly fell back to per-schema EXISTS checks, but PostgreSQL still logged an UndefinedFunction error every 500ms.

That made the fallback effectively noisy forever even though the absence of the helper is a stable condition for the lifetime of the process.

Fix

When the fast-path query fails specifically with asyncpg.exceptions.UndefinedFunctionError, mark the helper as unavailable for the current worker process and skip future probes. Other exception types still fall back without disabling future attempts, preserving the existing behavior for transient failures.

Testing

• uv run pytest tests/test_worker.py::TestClaimBatchRotation::test_scan_caches_missing_server_side_pending_work_function -v
• uv run ruff check hindsight_api/worker/poller.py tests/test_worker.py
• ./scripts/hooks/lint.sh

Impact

This removes persistent PostgreSQL log spam in deployments that do not install the optional helper function, without changing task claiming semantics.

[JulienJBO]

Closing this draft while local validation is still in progress. I will reopen or submit a fresh PR once tests and Sonar checks are complete.