fix: clean up Phase 1/2 commit issues

- Remove duplicate HTTP_STATUS/NETWORK exports in constants.js
- Remove unused responseHelpers import from server.js
- Remove backup file and doc artifacts from tracking
- Update .gitignore to prevent re-adding bloat files

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

dashcaddy-api/server.js

@@ -60,7 +60,6 @@ const auditLogger = require('./audit-logger');
 const portLockManager = require('./port-lock-manager');
 const dockerSecurity = require('./docker-security');
 const authManager = require('./auth-manager');
-const responseHelpers = require('./response-helpers');
 const configureMiddleware = require('./middleware');
 const { validateStartupConfig, syncHealthCheckerServices } = require('./startup-validator');
 const { CSRF_HEADER_NAME } = require('./csrf-protection');