harden(db): case-insensitive email unique index + dup-email test; list_users pagination TODO; from_db note

crates/domain/src/user.rs

@@ -56,6 +56,9 @@ impl Email {
     }
 
     /// Reconstruct from a stored (already-validated) value, without re-validating.
+    /// For reading values back from the database only — never to construct an `Email`
+    /// destined to be written (writes must go through [`Email::parse`] so storage
+    /// stays normalized).
     pub fn from_db(value: String) -> Email {
         Email(value)
     }