fix: don't join Drop cleanup thread — prevents deadlock in tokio tests

The Drop impl spawned a thread with a new tokio runtime and called
.join(), which blocked the test thread. The spawned thread's block_on
deadlocked when pool.close() tried to communicate with connections
owned by the outer tokio runtime. Removing .join() makes cleanup
fire-and-forget, avoiding the deadlock.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/tests/common/mod.rs

@@ -475,8 +475,9 @@ impl TestApp {
 
 impl Drop for TestApp {
     fn drop(&mut self) {
-        // Best-effort synchronous cleanup. The `cleanup()` async method is
-        // preferred, but this catches cases where it wasn't called.
+        // Fire-and-forget cleanup. Don't .join() — that deadlocks when
+        // running inside a tokio runtime (the spawned thread's block_on
+        // conflicts with the existing runtime's connection pool).
         let admin_pool = self.admin_pool.clone();
         let db_name = self.db_name.clone();
         let test_pool = self.pool.clone();
@@ -502,8 +503,7 @@ impl Drop for TestApp {
                     .execute(format!("DROP DATABASE IF EXISTS \"{}\"", db_name).as_str())
                     .await;
             });
-        })
-        .join()
-        .ok();
+        });
+        // Don't join — let the cleanup thread run independently
     }
 }