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/api/mailboxes.py

@@ -0,0 +1,37 @@
+"""Mailbox (folder) listing (SPEC §9.3 folder views, archive action)."""
+
+from __future__ import annotations
+
+import uuid
+from typing import Annotated
+
+from fastapi import APIRouter, Depends
+from pydantic import BaseModel
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.db import get_session
+from app.identity.deps import CurrentUser
+from app.models import Mailbox
+
+router = APIRouter(prefix="/mailboxes", tags=["mail"])
+
+DbSession = Annotated[AsyncSession, Depends(get_session)]
+
+
+class MailboxOut(BaseModel):
+    id: uuid.UUID
+    account_id: uuid.UUID
+    path: str
+    special_use: str | None
+
+
+@router.get("", response_model=list[MailboxOut])
+async def list_mailboxes(user: CurrentUser, db: DbSession) -> list[MailboxOut]:
+    rows = await db.scalars(
+        select(Mailbox).where(Mailbox.user_id == user.id).order_by(Mailbox.account_id, Mailbox.path)
+    )
+    return [
+        MailboxOut(id=m.id, account_id=m.account_id, path=m.path, special_use=m.special_use)
+        for m in rows
+    ]