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/search/factory.py

@@ -0,0 +1,21 @@
+"""Select the active SearchBackend from config (SPEC §8.1, env SEARCH_BACKEND)."""
+
+from __future__ import annotations
+
+from app.config import get_settings
+from app.search.base import SearchBackend
+from app.search.pg import PgSearchBackend
+
+_backend: SearchBackend | None = None
+
+
+def get_search_backend() -> SearchBackend:
+    """Return the process-wide SearchBackend (``pg`` in M0-M1, ``meili`` in M2)."""
+    global _backend
+    if _backend is None:
+        which = get_settings().search_backend
+        if which == "meili":
+            # TODO(spec): Meilisearch backend lands in M2.
+            raise NotImplementedError("meili backend is implemented in M2")
+        _backend = PgSearchBackend()
+    return _backend