Add gitleaks config to allowlist test files

Ignores backend/tests/*.py across all git history to avoid false
positives on variable names like 's3_key' in test assertions.

.gitleaks.toml

@@ -0,0 +1,8 @@
+# Gitleaks configuration
+# https://github.com/gitleaks/gitleaks#configuration
+
+[allowlist]
+# Test files that contain variable names matching secret patterns (e.g., s3_key)
+paths = [
+  '''backend/tests/.*\.py''',
+]