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/sync/supervisor.py

@@ -0,0 +1,61 @@
+"""Live-sync supervisor (SPEC §6.3).
+
+Runs inside the arq worker: on startup it launches one IDLE/poll loop per live
+account so new mail and flag changes flow to the UI over WebSocket. Loops
+self-heal (reconnect with backoff) and are cancelled on worker shutdown.
+
+NOTE(spec): accounts that become live *after* worker startup are picked up on the
+next worker restart; reacting to ``account.state_changed`` live is a later refinement.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import contextlib
+import logging
+import uuid
+from typing import Any
+
+from sqlalchemy import select
+
+from app.db import session_scope
+from app.models import Account
+from app.sync.engine import live_loop
+
+logger = logging.getLogger(__name__)
+
+_RECONNECT_BACKOFF = 30.0
+
+
+async def _run_account(account_id: uuid.UUID, stop: asyncio.Event) -> None:
+    while not stop.is_set():
+        try:
+            await live_loop(account_id, "INBOX", stop)
+        except asyncio.CancelledError:
+            raise
+        except Exception:
+            logger.exception("live loop for account %s crashed; reconnecting", account_id)
+            with contextlib.suppress(TimeoutError):
+                await asyncio.wait_for(stop.wait(), timeout=_RECONNECT_BACKOFF)
+
+
+async def start_supervisor(ctx: dict[str, Any]) -> None:
+    """Launch a live loop per live account (called from the worker on_startup)."""
+    async with session_scope() as session:
+        account_ids = list(await session.scalars(select(Account.id).where(Account.state == "live")))
+    stop = asyncio.Event()
+    ctx["supervisor_stop"] = stop
+    ctx["supervisor_tasks"] = [asyncio.create_task(_run_account(aid, stop)) for aid in account_ids]
+    logger.info("live supervisor started for %d account(s)", len(account_ids))
+
+
+async def stop_supervisor(ctx: dict[str, Any]) -> None:
+    """Signal and cancel all live loops (called from the worker on_shutdown)."""
+    stop = ctx.get("supervisor_stop")
+    if stop is not None:
+        stop.set()
+    tasks = ctx.get("supervisor_tasks", [])
+    for task in tasks:
+        task.cancel()
+    if tasks:
+        await asyncio.gather(*tasks, return_exceptions=True)