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/main.py

@@ -0,0 +1,45 @@
+"""FastAPI application factory (SPEC §2 ``app/main.py``)."""
+
+from __future__ import annotations
+
+from collections.abc import AsyncIterator
+from contextlib import asynccontextmanager
+
+from fastapi import FastAPI
+from sqlalchemy import text
+
+from app.config import get_settings
+from app.db import dispose_engine, get_engine
+from app.redis_client import close_redis, get_redis
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI) -> AsyncIterator[None]:
+    """Warm shared resources on startup, dispose them on shutdown."""
+    # Touch settings early so misconfiguration fails fast.
+    get_settings()
+    yield
+    await close_redis()
+    await dispose_engine()
+
+
+def create_app() -> FastAPI:
+    """Construct the FastAPI application."""
+    app = FastAPI(title="Webmail", version="0.1.0", lifespan=lifespan)
+
+    from app.api.errors import register_error_handlers
+
+    register_error_handlers(app)
+
+    @app.get("/api/healthz")
+    async def healthz() -> dict[str, str]:
+        """Liveness/readiness probe: checks DB and Redis (dev-setup §6)."""
+        async with get_engine().connect() as conn:
+            await conn.execute(text("SELECT 1"))
+        await get_redis().ping()
+        return {"status": "ok"}
+
+    from app.api import register_routers
+
+    register_routers(app)
+    return app