feat(handler): Phase 3G — multipart MIME, attachments, folder-per-email

[smallchungus]

Phase 3G: cloudhq-equivalent output.

• internal/handler/mime.go: parseMessage walks the MIME tree, extracts HTML/text body parts and attachments (base64/quoted-printable decoding included).
• Attachments written to /attachments// by RenderHandler.
• render.go emits meta.json with subject, from_email, received_at, attachment_names — cross-stage data for upload.
• upload.go reads meta.json, ensures per-email folder named _, uploads email.pdf + each attachment.
• Path helpers (SubjectSlug, FromSlug, EmailFolderName, SanitizeFilename) — pure unit-tested.
• Final Drive layout: /2026/04/18/2026-04-18_103045_re-project-update_alice/{email.pdf, report.pdf, ...}

Now genuinely cloudhq-equivalent for emails (minus OCR, inline-image embedding, exotic multipart/related cases — explicitly out of scope).