⚡ Bolt: Replace func.count() with limit(1) for user existence check

[ToolchainLab]

💡 What: Replaced func.count() with select(User.id).limit(1) in _is_bootstrap_admin in src/h4ckath0n/auth/service.py.
🎯 Why: Using func.count() on the entire User table requires scanning all rows or the entire index to compute the count, which is an O(N) operation. For a simple existence check (checking if the first user is being created), we only need to know if any row exists, making limit(1) an O(1) operation.
📊 Impact: Changes a potentially expensive full table/index scan to a highly optimized O(1) index lookup.
🔬 Measurement: Observe the database query execution plan and timing for the /auth/register route, expecting constant query execution time regardless of the total number of users.

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PR created automatically by Jules for task 8740286740998227850 started by @ToolchainLab

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