fix: count ALL open issues for spam multiplier instead of lookback-bounded subset (#929)

[alpurkan17]

Summary

run_mirror_issue_discovery derives open_issue_count from the same lookback-bounded /issues response used for scoring. Open issues created before PR_LOOKBACK_DAYS (35 days) are invisible, so the calculate_open_issue_spam_multiplier gate never penalizes miners with many old open issues.

This fix makes a second mirror API call without the since filter to count ALL open issues across mirror-enabled repos for the spam multiplier, while keeping the lookback-bounded call for scoring.

Validation

• ruff check --fix and ruff format pass
• Miners with open issues only older than the lookback window are now correctly penalized
• Falls back to lookback-bounded count if the unfiltered call fails
• Scoring remains lookback-bounded (unchanged)

Closes #929

[anderdc]

Duplicate of #930 and contribution quality is poor