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/base.py

@@ -0,0 +1,58 @@
+# INTERFACE VERSION: 1
+"""SearchBackend — the frozen search contract (SPEC §5.2).
+
+Both backends (``pg.py`` for M0-M1, ``meili.py`` for M2) index the same
+:class:`SearchDoc` and honour invariant I-7 (every query carries ``user_id``).
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from typing import Protocol
+from uuid import UUID
+
+
+@dataclass(frozen=True)
+class SearchQuery:
+    text: str | None  # free-text remainder after operator extraction
+    filters: dict[str, object]  # parsed operators, keys per §8.2
+    user_id: UUID  # ALWAYS set (invariant I-7)
+
+
+@dataclass(frozen=True)
+class SearchHit:
+    message_id: UUID
+    thread_id: UUID
+    score: float
+    highlights: dict[str, str]  # field -> HTML-escaped snippet with <mark>
+
+
+@dataclass(frozen=True)
+class SearchDoc:
+    """The document indexed by every backend (SPEC §5.2)."""
+
+    message_id: UUID
+    thread_id: UUID
+    user_id: UUID
+    account_id: UUID
+    mailbox_special_use: str | None
+    from_: list[dict[str, str]]  # name+email
+    to: list[dict[str, str]]
+    cc: list[dict[str, str]]
+    subject: str
+    body_text: str
+    attachment_names: list[str] = field(default_factory=list)
+    has_attachments: bool = False
+    is_unread: bool = False
+    is_starred: bool = False
+    date: int = 0  # unix seconds
+    size_bytes: int = 0
+
+
+class SearchBackend(Protocol):
+    async def index(self, doc: SearchDoc) -> None: ...
+    async def delete(self, message_id: UUID) -> None: ...
+    async def delete_user(self, user_id: UUID) -> None: ...
+    async def query(
+        self, q: SearchQuery, *, limit: int, cursor: str | None
+    ) -> tuple[list[SearchHit], str | None]: ...