feat: add admin endpoints for webhook replay and PR/issue refetch

[Kelvinchen03]

Summary

Adds admin endpoints for webhook replay and manual data refetch to recover from handler failures and data drift. Extends webhook_deliveries table to store payloads and track errors, enabling 30-day replay capability.

Related Issues

Closes #99

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

1. Applied database migration: 11_webhook_deliveries_extended.sql
2. Tested webhook failure tracking - verified failed_at and last_error populated
3. Tested replay endpoint - successfully reprocessed failed webhook delivery
4. Tested PR refetch endpoint - verified jobs enqueued and scoring_data_stored reset
5. Verified payload retention logic (7 days without payload, 30 days with payload)

Checklist

• I have read the Contributing Guide
• Code builds without errors
• New and existing tests pass (if applicable)
• Documentation updated (if applicable)
• No unnecessary dependencies added