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/threading_/normalize.py

@@ -0,0 +1,66 @@
+"""Message-ID and subject normalization (SPEC §7.1, §7.3)."""
+
+from __future__ import annotations
+
+import hashlib
+import re
+
+# Repeated reply/forward prefixes to strip, case-insensitive (incl. Polish Odp:/PD:).
+_PREFIX_RE = re.compile(r"^\s*(re|fw|fwd|odp|pd)\s*:\s*", re.IGNORECASE)
+_WS_RE = re.compile(r"\s+")
+_ANGLE_RE = re.compile(r"<([^>]*)>")
+
+
+def normalize_message_id(raw: str | None) -> str | None:
+    """Normalize a Message-ID per SPEC §7.1.
+
+    Strip angle brackets and whitespace, lowercase the domain part only.
+    Return None for obviously invalid ids (no ``@``).
+    """
+    if raw is None:
+        return None
+    text = raw.strip()
+    m = _ANGLE_RE.search(text)
+    if m:
+        text = m.group(1)
+    text = text.strip().strip("<>").strip()
+    if "@" not in text:
+        return None
+    local, _, domain = text.rpartition("@")
+    if not local or not domain:
+        return None
+    return f"{local}@{domain.lower()}"
+
+
+def normalize_reference_list(raw: str | None) -> list[str]:
+    """Parse a References / In-Reply-To header value into normalized ids, in order."""
+    if not raw:
+        return []
+    out: list[str] = []
+    for token in _ANGLE_RE.findall(raw) or raw.split():
+        norm = normalize_message_id(token)
+        if norm and norm not in out:
+            out.append(norm)
+    return out
+
+
+def synthesize_message_id(raw_message: bytes) -> str:
+    """Synthesize a non-authoritative Message-ID for a message that lacks one (§7.1)."""
+    digest = hashlib.sha256(raw_message).hexdigest()[:32]
+    return f"{digest}@synthetic.local"
+
+
+def is_synthetic(message_id: str | None) -> bool:
+    """Return True if ``message_id`` is a synthesized (non-authoritative) id."""
+    return message_id is not None and message_id.endswith("@synthetic.local")
+
+
+def normalize_subject(subject: str) -> str:
+    """Normalize a subject for the §7.3 fallback: strip repeated Re:/Fw:/... prefixes."""
+    text = subject or ""
+    while True:
+        stripped = _PREFIX_RE.sub("", text, count=1)
+        if stripped == text:
+            break
+        text = stripped
+    return _WS_RE.sub(" ", text).strip()