test: add service layer unit tests for webhook/metadata/error/config

- webhook_service_test.go: isPrivateIP, isSafeURL, computeHMAC
- request_metadata_test.go: context functions
- classified_error_test.go: error types
- config_defaults_test.go: password reset/SMS defaults
- email_config_test.go: email code defaults
- auth_runtime_test.go: isUserNotFoundError

Service coverage: 11.2% -> 14.7%

.claude/settings.local.json

@@ -67,7 +67,13 @@
       "Bash(while read:*)",
       "Bash(do basename:*)",
       "Bash(dir \"D:\\\\project\\\\frontend\")",
-      "Bash(grep -E '\\\\.txt$')"
+      "Bash(grep -E '\\\\.txt$')",
+      "Bash(go tool:*)",
+      "Bash(sort -t: -k2 -rn)",
+      "Bash(sort -t: -k3 -rn)",
+      "Bash(gosec ./...)",
+      "Bash(gosec -no-fail ./internal/...)",
+      "Bash(gosec -no-fail -quiet ./internal/...)"
     ]
   }
 }