feat(worker): add exponential idle backoff for empty task claims

[mfittko]

Summary

The enrichment worker currently polls /internal/tasks/claim every 1s per worker when the queue is empty. This creates unnecessary request/log noise at idle.

Problem

• Idle traffic scales with WORKER_CONCURRENCY (currently default 4).
• Empty-queue polling is constant and noisy in API logs.
• Current behavior does not adapt to prolonged idle periods.

Proposed change

Implement exponential idle backoff in the worker claim loop:

• Start at 1s after an empty claim.
• Increase on consecutive empty claims: 1s -> 2s -> 4s -> 8s ...
• Cap at 30-60s (choose one explicit cap).
• Reset backoff to 1s immediately after a successful claim.
• Optionally apply small jitter (recommended) to avoid synchronized polling across workers.

Acceptance criteria

• Worker claim loop uses exponential idle backoff for consecutive empty claims.
• Backoff resets to minimum after a successful claim.
• Maximum idle wait is capped at a configured constant (within 30-60s).
• Existing task-processing behavior and retries remain unchanged.
• Unit tests cover backoff increase, cap behavior, and reset-on-success.

Notes

This keeps the existing pull/claim queue model (no architecture change to webhook subscriber) while substantially reducing idle load.