refactor(application): use UniqueViolation in register — remove postgres constraint strings

crates/application/src/use_cases/auth.rs

@@ -38,11 +38,15 @@ pub async fn register(
         .save(&user)
         .await
         .map_err(|e| match e {
-            DomainError::Conflict(c) => match c.as_str() {
-                "users_username_key" => DomainError::Conflict("username taken".into()),
-                "users_email_key" => DomainError::Conflict("email taken".into()),
-                _ => DomainError::Conflict("already exists".into()),
-            },
+            DomainError::UniqueViolation { field: "username" } => {
+                DomainError::Conflict("username taken".into())
+            }
+            DomainError::UniqueViolation { field: "email" } => {
+                DomainError::Conflict("email taken".into())
+            }
+            DomainError::UniqueViolation { .. } => {
+                DomainError::Conflict("already exists".into())
+            }
             other => other,
         })?;
     events
@@ -136,7 +140,7 @@ mod tests {
     #[async_trait]
     impl UserWriter for ConflictOnSaveStore {
         async fn save(&self, _user: &User) -> Result<(), DomainError> {
-            Err(DomainError::Conflict("users_username_key".into()))
+            Err(DomainError::UniqueViolation { field: "username" })
         }
         async fn update_profile(
             &self,
@@ -178,7 +182,7 @@ mod tests {
     #[async_trait]
     impl UserWriter for EmailConflictOnSaveStore {
         async fn save(&self, _user: &User) -> Result<(), DomainError> {
-            Err(DomainError::Conflict("users_email_key".into()))
+            Err(DomainError::UniqueViolation { field: "email" })
         }
         async fn update_profile(
             &self,