fix: use two threads in testing to avoid deadlock

Unclear where this deadlock might be occurring

src/webgwas_backend/test_main.py

@@ -134,7 +134,7 @@ def client():
         dry_run=True,
         s3_bucket="TEST",
         sqlite_db=":memory:",
-        n_workers=1,
+        n_workers=2,
         indirect_gwas=IndirectGWASSettings(batch_size=10000),
     )
     worker = Worker(settings)