[NEEDS CODE REVIEWER] fix: Skip stale queue items during unmonitored checks

[jrhager84]

Summary

Prevents a stale queue item from crashing Decluttarr when its referenced detail item no longer exists in the configured Arr instance.

• Treats a 404 returned while checking monitored status as a stale queue record
• Logs which download and detail item were skipped
• Continues processing the remaining queue
• Re-raises non-404 HTTP errors instead of hiding unrelated failures
• Preserves the existing shared-download behavior by treating the stale record as non-removable

Closes #341

Test plan

• Stale queue item returning 404 is skipped
• Remaining unmonitored items are still detected
• Warning identifies the stale download and detail item
• Non-404 HTTP errors are re-raised
• pytest tests/jobs/test_remove_unmonitored.py tests/settings/test_instances.py tests/jobs/test_job_manager.py
• Black and isort checks

[lolimmlost]

Treating stale 404s as "monitored" is the right safe default -- preserves the shared-download guard (won't remove unless ALL queue items are verified unmonitored). Re-raise on non-404 is correct.

Non-blocking: make_request logs at ERROR before re-raising, so a stale 404 produces both an ERROR from make_request and a WARNING from the new handler. The ERROR is misleading since the situation is handled. Not worth changing is_monitored for this PR, but worth noting if the noise becomes a problem.

LGTM.