Webhooks: unified Resend inbound and delivery events on a single webhook endpoint

Features

• Webhooks: unified Resend inbound and delivery events on a single webhook endpoint
• Inbound: structured logging across the entire inbound email pipeline
• Docs: agent skills reference published on the website

Fixes

• Inbound: fallback reply-to-request linking when In-Reply-To header lookup misses
• Send: fallback body generation when payload lacks html or text
• Webhooks: fetch full email from Resend API on inbound receive
• Handle Resend headers as object instead of array
• Worker: unblock upsert by adding non-partial unique conflict index