fix: guard created_by FK in user creation handler

The auth identity user_id (from owner_id scope) may not match any
user row in the DB, causing a FK violation on the created_by column.
Check that the referenced user exists before setting created_by.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

src/channels/web/handlers/users.rs

@@ -61,7 +61,10 @@ pub async fn users_create_handler(
         created_at: now,
         updated_at: now,
         last_login_at: None,
-        created_by: Some(user.user_id.clone()),
+        created_by: match store.get_user(&user.user_id).await {
+            Ok(Some(_)) => Some(user.user_id.clone()),
+            _ => None,
+        },
         metadata: serde_json::json!({}),
     };