tests: use soulsync-testdb- prefix in the web_server endpoint tests

My credential/admin-gating endpoint tests override DATABASE_PATH at import to a
temp dir, but with a custom prefix — which tripped the DB-isolation guard
(test_database_path_env_is_isolated requires 'soulsync-testdb-' in the path) once
those modules loaded. Still a temp DB (no real-DB risk), just the wrong prefix.
Switched to the soulsync-testdb- prefix so isolation is preserved and the guard
passes.

tests/test_admin_gating.py

@@ -16,7 +16,7 @@ import tempfile
 
 import pytest
 
-_TMP = tempfile.mkdtemp(prefix='ss-admin-gate-')
+_TMP = tempfile.mkdtemp(prefix='soulsync-testdb-gate-')
 os.environ['DATABASE_PATH'] = os.path.join(_TMP, 'gate.db')
 os.environ['SOULSYNC_TEST_DB_READY'] = '1'
 

tests/test_credentials_endpoints.py

@@ -17,7 +17,7 @@ import tempfile
 import pytest
 
 # Redirect the DB before importing web_server so it never touches a real library.
-_TMP = tempfile.mkdtemp(prefix='ss-cred-ep-')
+_TMP = tempfile.mkdtemp(prefix='soulsync-testdb-cred-')
 os.environ['DATABASE_PATH'] = os.path.join(_TMP, 'creds_ep.db')
 os.environ['SOULSYNC_TEST_DB_READY'] = '1'