Add JS linting and deploy gating to CI pipeline

- Add js-lint job (ESLint 8) for static/ directory
- Add deploy job gated on both python-lint and js-lint passing
- Deploy triggers gandalf-deploy webhook on main branch only
- Add static/.eslintrc.json with browser env config
- Add node_modules/ to .gitignore

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

package.json

@@ -0,0 +1,5 @@
+{
+  "devDependencies": {
+    "eslint": "^8.57.1"
+  }
+}