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/0004_thread_pinned.py

@@ -0,0 +1,28 @@
+"""add threads.pinned
+
+Denormalized "pinned" state for a thread (SPEC extension for the UI pin action):
+true when any message in the thread carries the \\Flagged flag.
+
+Revision ID: 0004
+Revises: 0003
+Create Date: 2026-07-17
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+from alembic import op
+
+revision: str = "0004"
+down_revision: str | None = "0003"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.execute("ALTER TABLE threads ADD COLUMN pinned boolean NOT NULL DEFAULT false")
+
+
+def downgrade() -> None:
+    op.execute("ALTER TABLE threads DROP COLUMN IF EXISTS pinned")