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/schemas.py

@@ -0,0 +1,91 @@
+"""Shared API response models (SPEC §9.1)."""
+
+from __future__ import annotations
+
+import datetime as dt
+import uuid
+
+from pydantic import BaseModel
+
+
+class Address(BaseModel):
+    name: str = ""
+    email: str = ""
+
+
+class ThreadSummary(BaseModel):
+    id: uuid.UUID
+    subject: str
+    snippet: str
+    participants: list[Address]
+    account_ids: list[uuid.UUID]
+    message_count: int
+    unread_count: int
+    has_attachments: bool
+    pinned: bool
+    last_message_at: dt.datetime
+
+
+class ThreadListOut(BaseModel):
+    threads: list[ThreadSummary]
+    cursor: str | None = None
+
+
+class AttachmentOut(BaseModel):
+    id: uuid.UUID
+    filename: str
+    mime_type: str
+    size_bytes: int
+
+
+class MessageEnvelope(BaseModel):
+    id: uuid.UUID
+    account_id: uuid.UUID
+    mailbox_id: uuid.UUID
+    thread_id: uuid.UUID | None
+    subject: str
+    from_addr: list[Address]
+    to_addr: list[Address]
+    cc_addr: list[Address]
+    date: dt.datetime
+    flags: list[str]
+    is_unread: bool
+    is_starred: bool
+    has_attachments: bool
+    has_remote_images: bool
+    snippet: str
+
+
+class ThreadDetail(BaseModel):
+    id: uuid.UUID
+    subject: str
+    order: str  # "asc" | "desc"
+    pinned: bool
+    messages: list[MessageEnvelope]  # always ASC (SPEC §9.4)
+
+
+class FlagChangeRequest(BaseModel):
+    add: list[str] = []
+    remove: list[str] = []
+
+
+class MoveRequest(BaseModel):
+    mailbox_id: uuid.UUID
+
+
+class AccountOut(BaseModel):
+    id: uuid.UUID
+    email: str
+    display_name: str
+    color: str
+    state: str
+
+
+class SearchThreadHit(BaseModel):
+    thread: ThreadSummary
+    highlights: dict[str, str]
+
+
+class SearchOut(BaseModel):
+    threads: list[SearchThreadHit]
+    cursor: str | None = None