feat: support draft editing and file-sourced draft bodies

[addisonkline]

Summary

Two enhancements to message drafts in MAIL:

1. mail compose --body-file — the draft body can now be read from a file via -F/--body-file PATH as an alternative to the inline positional body. Exactly one of the two must be supplied.
2. PATCH /drafts/{draft_id} — a new endpoint letting an authenticated user-agent update an existing draft's subject, body, reply_to, and tags. Only supplied fields change (tags: [] clears, omitted leaves unchanged), and updated_at is refreshed on any edit. Exposed in the CLI as mail draft-edit (alias de).

Changes

• Protocol: DraftPatchRequest (all-optional fields) + DraftPatchResponse.
• Server: validate_patch_draft_request validator, PATCH /drafts/{draft_id} route (mirrors the patch_webhook precedent), and patch_draft on the backend base + memory impl (model_copy + updated_at, 404 on unknown/foreign draft).
• CLI: shared _body.py body resolver; optional positional body for compose; new draft-edit command.
• Spec/docs: regenerated spec/openapi.yaml; updated http.md, cli.md, and quickstart.md.

Design notes

• Per the answered design questions: --body-file is an explicit flag (not a @path/auto-detect convention), and PATCH does not guard against editing already-sent drafts (sent_at is never set under current behavior, so the guard couldn't trigger).
• reply_to is editable via PATCH, matching compose.

Testing

• 7 new integration tests (update / partial / clear-tags / 422 / 404 / cross-user isolation / edit-then-send) and 4 new client-command tests (file body, both/neither errors, draft-edit request shape); updated existing compose tests for the new body_file arg.
• Full suite: 443 passed, 6 xfailed (xfails are pre-existing NotImplementedError stubs). ruff clean on all touched files; OpenAPI drift test passes.

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