⚡ Bolt: Use db.scalar() instead of db.execute().scalars().first() for database fetches

[ToolchainLab]

💡 What: Refactored db.execute(stmt).scalars().first() to db.scalar(stmt). Also updated one query to only fetch the primary key (db.scalar(select(User.id)...)) when checking for existence.
🎯 Why: Prevents unnecessary query execution and iteration steps, which provides small but frequent wins in hot paths like auth. Fetching only the primary key avoids hydrating the entire ORM object unnecessarily.
📊 Impact: Reduces SQLAlchemy overhead and memory allocations during database fetches.
🔬 Measurement: Profiling .execute().scalars().first() vs .scalar() shows a measurable reduction in CPU time when querying single records.

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PR created automatically by Jules for task 10190036784636829498 started by @ToolchainLab

[google-labs-jules]

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