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/migrations/versions/0002_user_msgid_index.py

@@ -0,0 +1,28 @@
+"""index messages(user_id, message_id_hdr)
+
+Speeds the per-user Message-ID lookups used by JWZ threading (§7.2 candidate
+set) and by the duplicate-Message-ID resolution in the indexer (§7.1).
+
+Revision ID: 0002
+Revises: 0001
+Create Date: 2026-07-17
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+from alembic import op
+
+revision: str = "0002"
+down_revision: str | None = "0001"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.execute("CREATE INDEX ix_messages_user_msgid ON messages (user_id, message_id_hdr)")
+
+
+def downgrade() -> None:
+    op.execute("DROP INDEX IF EXISTS ix_messages_user_msgid")