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/errors.py

@@ -0,0 +1,35 @@
+"""Consistent API error shape ``{error:{code,message}}`` (SPEC §9.1)."""
+
+from __future__ import annotations
+
+from typing import NoReturn
+
+from fastapi import FastAPI, HTTPException, Request
+from fastapi.responses import JSONResponse
+from starlette.exceptions import HTTPException as StarletteHTTPException
+
+
+def api_error(status_code: int, code: str, message: str) -> HTTPException:
+    """Build an :class:`HTTPException` carrying the ``{code,message}`` detail."""
+    return HTTPException(status_code=status_code, detail={"code": code, "message": message})
+
+
+def raise_api_error(status_code: int, code: str, message: str) -> NoReturn:
+    """Raise :func:`api_error` (typed as ``NoReturn`` for control-flow clarity)."""
+    raise api_error(status_code, code, message)
+
+
+async def _handler(request: Request, exc: StarletteHTTPException) -> JSONResponse:
+    detail = exc.detail
+    if isinstance(detail, dict) and "code" in detail:
+        body = {"error": detail}
+    else:
+        body = {"error": {"code": "http_error", "message": str(detail)}}
+    headers = getattr(exc, "headers", None)
+    return JSONResponse(status_code=exc.status_code, content=body, headers=headers)
+
+
+def register_error_handlers(app: FastAPI) -> None:
+    """Install the error-shape handler for all HTTP exceptions."""
+    app.add_exception_handler(StarletteHTTPException, _handler)  # type: ignore[arg-type]
+    app.add_exception_handler(HTTPException, _handler)  # type: ignore[arg-type]