refactor: move profile-field count validation into domain UserProfile

crates/domain/src/models/tests/mod.rs

@@ -230,3 +230,26 @@ fn movie_is_manual_match_different_director_fails() {
     let year = crate::value_objects::ReleaseYear::new(1982).unwrap();
     assert!(!m.is_manual_match(&title, &year, Some("Denis Villeneuve")));
 }
+
+// ── UserProfile field validation ─────────────────────────────────────────────
+
+#[test]
+fn profile_fields_validates_max_count() {
+    let fields: Vec<ProfileField> = (0..5)
+        .map(|i| ProfileField { name: format!("f{i}"), value: format!("v{i}") })
+        .collect();
+    assert!(UserProfile::validate_custom_fields(&fields).is_err());
+}
+
+#[test]
+fn profile_fields_allows_four() {
+    let fields: Vec<ProfileField> = (0..4)
+        .map(|i| ProfileField { name: format!("f{i}"), value: format!("v{i}") })
+        .collect();
+    assert!(UserProfile::validate_custom_fields(&fields).is_ok());
+}
+
+#[test]
+fn profile_fields_allows_zero() {
+    assert!(UserProfile::validate_custom_fields(&[]).is_ok());
+}

crates/domain/src/models/user.rs

@@ -32,6 +32,20 @@ pub struct UserProfile {
     pub profile_fields: Vec<ProfileField>,
 }
 
+impl UserProfile {
+    pub const MAX_CUSTOM_FIELDS: usize = 4;
+
+    pub fn validate_custom_fields(fields: &[ProfileField]) -> Result<(), crate::errors::DomainError> {
+        if fields.len() > Self::MAX_CUSTOM_FIELDS {
+            Err(crate::errors::DomainError::ValidationError(
+                "Maximum 4 profile fields allowed".into(),
+            ))
+        } else {
+            Ok(())
+        }
+    }
+}
+
 #[derive(Clone, Debug)]
 pub struct User {
     id: UserId,