feat(users): normalize email identity storage

apps/users/email_identity.py

@@ -0,0 +1,16 @@
+from __future__ import annotations
+
+
+def normalize_email_identity(value: str | None) -> str | None:
+    if value is None:
+        return None
+    normalized = value.strip().lower()
+    return normalized or None
+
+
+def mask_mobile(value: str | None) -> str | None:
+    if not value:
+        return None
+    if len(value) <= 4:
+        return value
+    return f"{value[:2]}{'*' * max(len(value) - 6, 1)}{value[-4:]}"