⚡ Bolt: Optimize user existence check during registration

[ToolchainLab]

⚡ Bolt: Optimize user existence check during registration

💡 What: Replaced the full ORM hydration check for user existence (db.execute(select(User)).scalars().first()) with a targeted scalar check for just the primary key (db.scalar(select(User.id))).

🎯 Why: The original query forces SQLAlchemy to parse, allocate, and hydrate the entire User model instance just to check if an email is already registered. This is unnecessary overhead for a simple boolean existence check.

📊 Impact: Reduces database bandwidth and memory allocation overhead during user registration by fetching only a single UUID instead of the full user record.

🔬 Measurement: Run the test suite using uv run pytest to ensure registration flows and duplicate user constraints continue to function exactly as before.

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PR created automatically by Jules for task 9511693914332887515 started by @ToolchainLab

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