ci: restructure pipeline into parallel jobs with E2E, Docker validation, and auditing

- Split serial build-and-test job into parallel backend, frontend, docker-validate, and e2e jobs
- backend job: build → test → lint (new ESLint) → npm audit --audit-level=high
- frontend job: build → lint → npm audit --audit-level=high
- docker-validate job: builds image on every PR without pushing; Trivy scans for CRITICAL/HIGH CVEs (informational)
- e2e job: runs full Playwright suite against live dev servers after backend+frontend pass
- Add backend/eslint.config.mjs and lint script to backend/package.json

backend/package.json

@@ -7,7 +7,8 @@
     "build": "tsc",
     "start": "node dist/index.js",
     "dev": "nodemon --watch src --ext ts,json --exec ts-node src/index.ts",
-    "test": "vitest run"
+    "test": "vitest run",
+    "lint": "eslint src"
   },
   "keywords": [],
   "author": "",
@@ -26,7 +27,10 @@
     "supertest": "^7.2.2",
     "ts-node": "^10.9.2",
     "typescript": "^5.9.3",
-    "vitest": "^4.1.0"
+    "vitest": "^4.1.0",
+    "@eslint/js": "^9.0.0",
+    "eslint": "^9.0.0",
+    "typescript-eslint": "^8.0.0"
   },
   "dependencies": {
     "@types/cors": "^2.8.19",