fix: rollback queue counters for blacklisted child URLs

[harshang03]

Summary

• add queue rollback metadata markers for extractors that pre-increment counters for external links
• rollback queue counter fields when child queue entries are skipped by blacklist filtering
• wire rollback markers into Bellazon and XenForo external-link queue paths
• add job-level regression tests for filtered vs non-filtered queue handling

What changed

• gallery_dl/job.py: rollback queue counters on filtered and duplicate queue skips
• gallery_dl/extractor/message.py: define queue rollback metadata keys
• gallery_dl/extractor/common.py: helper to annotate queue rollback metadata
• gallery_dl/extractor/bellazon.py: mark external queue counters for rollback
• gallery_dl/extractor/xenforo.py: mark external queue counters for rollback
• test/test_job.py: add regression coverage for queue counter rollback

Test plan

• ./.venv/bin/python -m pytest test/test_job.py

Fixes #8513