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/tests/threading/test_normalize.py

@@ -0,0 +1,55 @@
+"""Message-ID and subject normalization (SPEC §7.1, §7.3)."""
+
+from __future__ import annotations
+
+from app.threading_.normalize import (
+    is_synthetic,
+    normalize_message_id,
+    normalize_reference_list,
+    normalize_subject,
+    synthesize_message_id,
+)
+
+
+def test_normalize_message_id_strips_brackets_and_lowercases_domain() -> None:
+    assert normalize_message_id("<Abc.123@Example.COM>") == "Abc.123@example.com"
+    assert normalize_message_id("  <x@Y.Z>  ") == "x@y.z"
+    # local part case is preserved, domain lowercased
+    assert normalize_message_id("<CaseSensitive@Domain.Com>") == "CaseSensitive@domain.com"
+
+
+def test_normalize_message_id_rejects_invalid() -> None:
+    assert normalize_message_id(None) is None
+    assert normalize_message_id("no-at-sign") is None
+    assert normalize_message_id("<>") is None
+    assert normalize_message_id("@nolocal.com") is None
+
+
+def test_normalize_reference_list_order_and_dedupe() -> None:
+    raw = "<a@x.com> <b@y.com> <a@X.com>"
+    assert normalize_reference_list(raw) == ["a@x.com", "b@y.com"]
+    assert normalize_reference_list("") == []
+    assert normalize_reference_list(None) == []
+
+
+def test_normalize_reference_list_whitespace_separated_without_brackets() -> None:
+    assert normalize_reference_list("a@x.com b@y.com") == ["a@x.com", "b@y.com"]
+
+
+def test_synthetic_id() -> None:
+    sid = synthesize_message_id(b"raw bytes here")
+    assert sid.endswith("@synthetic.local")
+    assert is_synthetic(sid)
+    assert not is_synthetic("real@example.com")
+    assert not is_synthetic(None)
+    # deterministic
+    assert synthesize_message_id(b"raw bytes here") == sid
+
+
+def test_normalize_subject_strips_prefixes() -> None:
+    assert normalize_subject("Re: Hello") == "Hello"
+    assert normalize_subject("RE: FW: Fwd:  Hello  world") == "Hello world"
+    assert normalize_subject("Odp: Cześć") == "Cześć"  # Polish reply prefix
+    assert normalize_subject("PD: X") == "X"
+    assert normalize_subject("No prefix") == "No prefix"
+    assert normalize_subject("") == ""