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/0005_compose_attachments.py

@@ -0,0 +1,53 @@
+"""add compose_attachments
+
+Files uploaded for outgoing mail (the compose "Attach" action). Referenced by a
+draft's payload.attachment_ids and assembled into the MIME at send time.
+
+Revision ID: 0005
+Revises: 0004
+Create Date: 2026-07-17
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0005"
+down_revision: str | None = "0004"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "compose_attachments",
+        sa.Column(
+            "id",
+            sa.dialects.postgresql.UUID(as_uuid=True),
+            primary_key=True,
+            server_default=sa.text("gen_random_uuid()"),
+        ),
+        sa.Column(
+            "user_id",
+            sa.dialects.postgresql.UUID(as_uuid=True),
+            sa.ForeignKey("users.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column("filename", sa.Text(), nullable=False),
+        sa.Column("mime_type", sa.Text(), nullable=False),
+        sa.Column("size_bytes", sa.Integer(), nullable=False),
+        sa.Column("blob_sha256", sa.Text(), nullable=False),
+        sa.Column(
+            "created_at",
+            sa.TIMESTAMP(timezone=True),
+            nullable=False,
+            server_default=sa.text("now()"),
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("compose_attachments")