Initial commit: webmail client (M0/M1)

Three-pane webmail client.
- Backend: FastAPI + async SQLAlchemy + arq worker (Postgres/Redis), IMAP/SMTP
  sync, threading, search, drafts/outbox with undo, attachments, 2FA.
- Frontend: React 18 + Vite + TypeScript + TanStack Query + Tailwind; three-pane
  UI with rich-text composer, light/dark themes, resizable panels.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/providers/imap_smtp/__init__.py

@@ -0,0 +1,7 @@
+"""IMAP+SMTP provider — the only M0-M2 MailProvider implementation (SPEC §5.1)."""
+
+from __future__ import annotations
+
+from app.providers.imap_smtp.provider import ImapSmtpProvider, ProviderConfig
+
+__all__ = ["ImapSmtpProvider", "ProviderConfig"]