feat: query AI checkpoint recovery candidates

[svarlamov]

Summary

• add one batched, bounded lookup for nearby AI checkpoint metrics
• retain original ai_agent and ai_tab checkpoints while excluding human, mock_ai, malformed, and recovery-generated events
• reuse the existing metrics_event_ts_kind index; no schema or index migration
• parse checkpoint path, session, tool/model, repository, and base-commit metadata for the recovery solver in the child PR

Tests

• filter and metadata parsing coverage, including zero-addition checkpoints
• EXPLAIN QUERY PLAN assertion that the lookup uses metrics_event_ts_kind without a metrics-table scan

[svarlamov]

• feat: recover attribution from AI checkpoints #1970
• feat: query AI checkpoint recovery candidates #1969 👈 (View in Graphite)
• main

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.