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/workers/__init__.py

@@ -0,0 +1 @@
+"""arq worker package (SPEC §2 workers/)."""

backend/workers/worker.py

@@ -0,0 +1,68 @@
+"""arq worker entrypoint (SPEC §2 ``workers/worker.py``).
+
+Run with: ``arq workers.worker.WorkerSettings`` (from ``backend/``).
+"""
+
+from __future__ import annotations
+
+import logging
+import uuid
+from typing import Any
+
+from app.config import get_settings
+from app.outbox.jobs import send_job
+from app.queue import redis_settings as arq_redis_settings
+from app.sync.actions import (
+    delete_thread_job,
+    move_job,
+    move_thread_to_folder_job,
+    push_flags_job,
+)
+from app.sync.engine import bootstrap_and_backfill
+
+logger = logging.getLogger(__name__)
+
+
+async def sync_bootstrap_account(ctx: dict[str, Any], account_id: str) -> int:
+    """Bootstrap + backfill an account (enqueued on account creation)."""
+    logger.info("sync.bootstrap_account start account=%s", account_id)
+    count = await bootstrap_and_backfill(uuid.UUID(account_id))
+    logger.info("sync.bootstrap_account done account=%s messages=%d", account_id, count)
+    return count
+
+
+async def on_startup(ctx: dict[str, Any]) -> None:
+    logging.basicConfig(level=get_settings().log_level)
+    from app.sync.supervisor import start_supervisor
+
+    await start_supervisor(ctx)
+    logger.info("webmail worker started")
+
+
+async def on_shutdown(ctx: dict[str, Any]) -> None:
+    from app.db import dispose_engine
+    from app.redis_client import close_redis
+    from app.sync.supervisor import stop_supervisor
+
+    await stop_supervisor(ctx)
+    await dispose_engine()
+    await close_redis()
+    logger.info("webmail worker stopped")
+
+
+class WorkerSettings:
+    """arq worker configuration."""
+
+    functions = [
+        sync_bootstrap_account,
+        push_flags_job,
+        move_job,
+        move_thread_to_folder_job,
+        delete_thread_job,
+        send_job,
+    ]
+    on_startup = on_startup
+    on_shutdown = on_shutdown
+    max_jobs = 10  # global worker cap (SPEC §6.3)
+    job_timeout = 3600  # backfill of a large account can be long
+    redis_settings = arq_redis_settings()