⚡ Bolt: Optimize existence checks and ORM hydration in auth service

[ToolchainLab]

• 💡 What: Replaced db.execute(select(func.count())...) and db.execute(select(Model)...).scalars().first() with db.scalar(select(Model.id)...limit(1)) and db.scalar(...) in src/h4ckath0n/auth/service.py.
• 🎯 Why: Using func.count() scans the whole table, and execute().scalars().first() for existence checks creates unnecessary ExecutionResult allocations and forces full ORM object hydration.
• 📊 Impact: Eliminates O(N) full table scans for initial admin checking and avoids full object hydration when we only care about user/token existence. Reduces database load and memory usage.
• 🔬 Measurement: Observe SQL queries generated during user registration and password resets, noticing LIMIT 1 instead of COUNT() and skipping full model load. Run uv run pytest to ensure correctness.

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PR created automatically by Jules for task 13045407719132276687 started by @ToolchainLab

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