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/db.py

@@ -0,0 +1,72 @@
+"""Async SQLAlchemy engine/session management (SPEC §2 ``app/db.py``)."""
+
+from __future__ import annotations
+
+from collections.abc import AsyncIterator
+from contextlib import asynccontextmanager
+
+from sqlalchemy.ext.asyncio import (
+    AsyncEngine,
+    AsyncSession,
+    async_sessionmaker,
+    create_async_engine,
+)
+from sqlalchemy.orm import DeclarativeBase
+
+from app.config import get_settings
+
+_engine: AsyncEngine | None = None
+_sessionmaker: async_sessionmaker[AsyncSession] | None = None
+
+
+class Base(DeclarativeBase):
+    """Declarative base for all ORM models."""
+
+
+def get_engine() -> AsyncEngine:
+    """Return (creating on first use) the process-wide async engine."""
+    global _engine
+    if _engine is None:
+        _engine = create_async_engine(
+            get_settings().database_url,
+            pool_pre_ping=True,
+            future=True,
+        )
+    return _engine
+
+
+def get_sessionmaker() -> async_sessionmaker[AsyncSession]:
+    """Return the process-wide session factory."""
+    global _sessionmaker
+    if _sessionmaker is None:
+        _sessionmaker = async_sessionmaker(
+            get_engine(), expire_on_commit=False, class_=AsyncSession
+        )
+    return _sessionmaker
+
+
+@asynccontextmanager
+async def session_scope() -> AsyncIterator[AsyncSession]:
+    """Transactional session context manager for workers and scripts."""
+    async with get_sessionmaker()() as session:
+        try:
+            yield session
+            await session.commit()
+        except Exception:
+            await session.rollback()
+            raise
+
+
+async def get_session() -> AsyncIterator[AsyncSession]:
+    """FastAPI dependency yielding a request-scoped session."""
+    async with get_sessionmaker()() as session:
+        yield session
+
+
+async def dispose_engine() -> None:
+    """Dispose the engine (used on app shutdown)."""
+    global _engine, _sessionmaker
+    if _engine is not None:
+        await _engine.dispose()
+        _engine = None
+        _sessionmaker = None