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/tests/threading/env.py

@@ -0,0 +1,87 @@
+"""Test helper: a user with two accounts/mailboxes + a message factory.
+
+Kept out of ``conftest.py`` so tests can import :class:`ThreadingEnv` without
+re-importing the conftest module.
+"""
+
+from __future__ import annotations
+
+import uuid
+from datetime import UTC, datetime, timedelta
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models import Account, Mailbox, Message, User
+from app.threading_.threader import thread_message
+
+BASE = datetime(2026, 1, 1, 12, 0, 0, tzinfo=UTC)
+
+
+class ThreadingEnv:
+    """A user with two accounts + one mailbox each, plus a message factory."""
+
+    def __init__(self, session: AsyncSession) -> None:
+        self.session = session
+        self.user_id: uuid.UUID
+        self.account_a: uuid.UUID
+        self.account_b: uuid.UUID
+        self.mailbox_a: uuid.UUID
+        self.mailbox_b: uuid.UUID
+        self._uid = 0
+
+    async def setup(self) -> None:
+        user = User(handle=f"t{uuid.uuid4().hex[:8]}@test.local", password_hash="x")
+        self.session.add(user)
+        await self.session.flush()
+        self.user_id = user.id
+        for label in ("a", "b"):
+            account = Account(
+                user_id=self.user_id,
+                email=f"{label}@example.com",
+                imap_config={},
+                smtp_config={},
+                secret_ct=b"",
+                discovery_src="manual",
+            )
+            self.session.add(account)
+            await self.session.flush()
+            mailbox = Mailbox(account_id=account.id, user_id=self.user_id, path="INBOX")
+            self.session.add(mailbox)
+            await self.session.flush()
+            setattr(self, f"account_{label}", account.id)
+            setattr(self, f"mailbox_{label}", mailbox.id)
+
+    async def add(
+        self,
+        *,
+        message_id: str | None,
+        subject: str = "Subject",
+        references: list[str] | None = None,
+        in_reply_to: str | None = None,
+        from_email: str = "alice@example.com",
+        to_email: str = "bob@example.com",
+        minutes: int = 0,
+        account: str = "a",
+        flags: list[str] | None = None,
+    ) -> Message:
+        self._uid += 1
+        msg = Message(
+            user_id=self.user_id,
+            account_id=getattr(self, f"account_{account}"),
+            mailbox_id=getattr(self, f"mailbox_{account}"),
+            uid=self._uid,
+            message_id_hdr=message_id,
+            in_reply_to=in_reply_to,
+            references_hdr=references or [],
+            subject=subject,
+            from_addr=[{"name": "", "email": from_email}],
+            to_addr=[{"name": "", "email": to_email}],
+            internal_date=BASE + timedelta(minutes=minutes),
+            flags=flags or [],
+            blob_sha256="0" * 64,
+            snippet=subject,
+        )
+        self.session.add(msg)
+        await self.session.flush()
+        await thread_message(self.session, msg)
+        return msg