⚡ Bolt: Optimize user existence check in bootstrap admin logic

[ToolchainLab]

💡 What: Replaced a select(func.count()) full table scan with a select(User.id).limit(1) check in the _is_bootstrap_admin logic.
🎯 Why: Using func.count() on large tables without WHERE clauses forces the database to scan all rows or a full index, causing a performance bottleneck. Checking for the existence of a single row using limit(1) allows the query to return immediately after finding the first record, making the check an O(1) operation.
📊 Impact: Existence check becomes O(1) instead of O(N), avoiding severe database locking or CPU overhead during initial user bootstrap checks on large databases.
🔬 Measurement: Run unit tests with pytest to ensure logic correctness. The optimization is visible in src/h4ckath0n/auth/service.py.

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PR created automatically by Jules for task 5802081264205584754 started by @ToolchainLab

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