⚡ Bolt: [performance improvement] Parallelize admin stats queries

[secrooq]

💡 What: Refactored the src/app/api/admin/[[...slug]]/route.ts endpoint to fetch admin overview statistics (files, emails, users, secrets) concurrently using Promise.all instead of awaiting them sequentially.

🎯 Why: The original implementation caused N+1 network latency by waiting for each Cloudflare D1 database roundtrip to finish before starting the next one.

📊 Impact: Reduces the TTFB (Time to First Byte) latency of the /api/admin/stats endpoint by parallelizing the four independent database queries, reducing the database latency overhead from $O(A+B+C+D)$ to $O(\max(A,B,C,D))$.

🔬 Measurement: Verified that the endpoint continues to return all correct aggregate statistics and handles individual query failures gracefully via isolated .catch handlers within the Promise.all array.

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PR created automatically by Jules for task 17722097455709645319 started by @secrooq

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