v0.10.1 — webhook payloads carry a readable body

Webhook payloads carried the message only as base64 raw.data. That's useless to the two things that actually receive these — an LLM can't reliably decode base64, and a script shouldn't have to — and it inflated every payload by a third, so a 100 KB mail arrived as 133 KB of unreadable characters.

Found by wiring the forwarder to a real agent rather than a test endpoint: it could read the subject and sender but not the body, so it correctly logged no-action on everything it was sent. The forwarding worked; the payload was the problem.

Payloads now include:

• text and html — the decoded bodies, reusing the same MIME walk the message routes already use
• attachments — filename, content type, size, disposition
• raw — still present, for receivers that would rather parse MIME themselves

342 tests, 341 passing, 1 skipped.