Internal PG Sync Webhook

POST /api/internal/pg-sync

• Purpose: Convex → Postgres durable sync webhook
• Auth: HMAC header x-rf-signature computed with RUNFORGE_SYNC_SIGN
• Body: JSON of a single runs_live doc (validated)

Notes - The OSS template can run without Postgres; this endpoint is a no‑op unless RUNFORGE_SYNC_URL and RUNFORGE_SYNC_SIGN are configured.