Fix/solved issue can become stale

[jonathanchang31]

Summary

This update improves mirror reliability and scoring correctness by fixing:

1. Webhook deduplication race for identical x-github-delivery.
2. Stale issues.solved_by_pr links after PR closing-reference changes.

Related Issue

Fixes: #59

Change Type (select all)

• Bug fix
• Concurrency/race-condition fix
• Data consistency fix
• Reliability improvement
• Backend logic update
• DB schema update
• Breaking change
• API contract change

Changes Implemented

1. Webhook delivery lease model:

• Added processing_started_at to webhook_deliveries.
• Atomic claim/reclaim query in claimDelivery().
• markProcessed() now clears lease.
• releaseDelivery() on handler failure to allow immediate retry.

2. solved_by_pr reconciliation:

• Read previous issue links before metadata update.
• Compute removed links and clear stale mappings.
• Set current links when PR is merged.
• Clear this PR as solver when PR is not merged.

Real Behavior Proof

Before:

1. Two concurrent webhook posts with same delivery ID could both execute handlers.
2. After merging PR with Fixes #123, editing PR body to remove link left issues.solved_by_pr still pointing to old PR.

After:

1. Only one request can hold active claim for a delivery; others skip unless stale-lease reclaim conditions are met.
2. Metadata refresh clears removed stale issue mappings and re-applies only valid current merged links.

[anderdc]

The webhook_deliveries lease / processing_started_at change conflicts with the documented design contract — webhook reprocessing is intentional, all handlers are upserts so same-delivery-id retries are safe, and the named non-idempotent side-effect (label_events) is already being fixed at the write layer in #26. Closing. Issue #59 (stale solved_by_pr) remains open — the reconciliation fix can land in a separate PR without the lease changes.