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/tests/threading/conftest.py

@@ -0,0 +1,15 @@
+"""Fixtures for threading tests: the shared rollback session + a threading env."""
+
+from __future__ import annotations
+
+import pytest_asyncio
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from tests.threading.env import ThreadingEnv
+
+
+@pytest_asyncio.fixture
+async def env(db_session: AsyncSession) -> ThreadingEnv:
+    e = ThreadingEnv(db_session)
+    await e.setup()
+    return e