fix: allowing unknown solvers through mirror solved-issue pipeline

[jonathanchang31]

Summary

This change fixes a scoring-integrity bug where backfill stored missing PR author IDs as empty strings ('') instead of NULL. Because solver selection only filtered IS NOT NULL, unknown solver identities leaked into solving_pr.

Related Issue

Fixes: #85

Change Type (select all)

• Bug fix
• Data integrity fix
• API response correctness fix
• Backfill pipeline correction
• DB data normalization script
• Breaking change
• New feature

Root Cause

• Backfill PR upsert used:
  • authorGithubId: String(pr.author?.databaseId ?? "")
• Empty string passed solver guard:
  • sp.author_github_id IS NOT NULL

Result: unknown solver IDs ('') were treated as present and returned in solved-issue pipeline.

Real Behavior Proof

Before

• Missing PR author in backfill -> author_github_id = ''
• Solver subquery accepted it (IS NOT NULL true)
• API could emit solved issues with unknown/blank solver identity

After

• Missing PR author in backfill -> author_github_id = NULL
• Solver subquery rejects both null and blank IDs
• Unknown solvers are excluded from solving_pr as intended

Checklist

• Fix implemented in backfill PR ingestion path
• Solver SQL guard hardened for blank values
• Legacy normalization SQL added
• Patch re-validated by code inspection
• Migration applied in target DB environment
• Tests run in CI/local