[infra] Add UNIQUE constraint on webhook_events.delivery_id (PRD PR-2) #43

Context

PRD v0.2 AC-F3.5 and PR-2 require deduplication via unique delivery_id on webhook_events.

Problem

migrations/001_init.sql stores delivery_id but has no UNIQUE constraint. Issue #9 covers application logic; schema must enforce at DB level.

Proposed scope

• Add migration 002_webhook_delivery_id_unique.sql with partial unique index on delivery_id WHERE delivery_id IS NOT NULL
• Handle conflict in gateway insert path

Acceptance criteria

• Duplicate X-GitHub-Delivery cannot insert second row
• Gateway returns idempotent success without second chain submission
• Migration is backward-safe for existing rows

References

• migrations/001_init.sql
• crates/gateway/src/attestation.rs (persist_webhook_event)
• docs/PRD.md (AC-F3.5, PR-2)