⚡ Bolt: Optimize single-model database fetches

[ToolchainLab]

💡 What:

• Refactored db.execute(select(...)).scalars().first() to use db.scalar(select(...)) across src/h4ckath0n/auth/service.py, src/h4ckath0n/auth/passkeys/service.py, and src/h4ckath0n/cli/_common.py.
• Refactored select(func.count()) in _is_bootstrap_admin to use select(User.id).limit(1).

🎯 Why:

• Calling db.execute(...).scalars().first() forces SQLAlchemy to allocate and parse intermediate ExecutionResult and ScalarResult objects, which is unnecessary overhead when only fetching a single item (or checking existence). Using db.scalar optimizes this.
• Using func.count() to check if a table is empty is an anti-pattern that can cause full table scans on large tables. Using limit(1) is a massive performance win for existence checks.

📊 Impact:

• Reduces memory allocation and parsing overhead on highly frequent operations (like authenticate_user and register_user).
• Prevents O(N) database operations in bootstrap checks, making it O(1).

🔬 Measurement:

• uv run pytest confirms that all auth and CLI flows behave identically.

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PR created automatically by Jules for task 9060013900320677031 started by @ToolchainLab

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