⚡ Bolt: Optimize empty table check in bootstrap admin logic

[ToolchainLab]

💡 What: Replaced select(func.count()).select_from(User) with select(User.id).limit(1) in the _is_bootstrap_admin logic.
🎯 Why: The previous logic performed a full table or index scan to count all users, just to check if the table was empty. As the user table grows, this query becomes increasingly slow on every single registration attempt.
📊 Impact: Converts an O(N) database operation into an O(1) operation. On large tables, this prevents registration latency spikes and reduces database load.
🔬 Measurement: Verified the improvement locally with artificial data payloads in sqlite/postgres benchmarks, showing immediate speedup. Verified backend tests and frontend E2E passkey tests pass with the new logic.

---

PR created automatically by Jules for task 5657861255228661968 started by @ToolchainLab

[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.