⚡ Bolt: [performance improvement] Optimize admin stats concurrent database queries

[secrooq]

💡 What: Modified src/app/api/admin/[[...slug]]/route.ts to replace 4 sequential database queries (vault_files, relay_aliases, vault_users, stealth_secrets) with concurrent execution using Promise.all. Individual catch handlers are preserved to ensure fault tolerance.
🎯 Why: In serverless Edge environments connecting to Cloudflare D1 via HTTP, each db.prepare().first() call incurs network latency (the N+1 problem). Executing them sequentially multiplies this latency. db.batch() treats them as a single transaction which fails if one query fails, so Promise.all is the correct approach to minimize roundtrips while preserving isolation.
📊 Impact: Reduces database roundtrip latency in the /api/admin/stats endpoint by ~75% (from $O(N)$ sequential network requests to $O(1)$ concurrent requests).
🔬 Measurement: Build successfully completed (pnpm run build), all E2E tests passing (npx playwright test). Response times on /api/admin/stats will be noticeably faster.

---

PR created automatically by Jules for task 4877960886567421283 started by @secrooq

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.