fix(auth): optimize first user check in create_profile

[BillionClaw]

Summary

Replaces inefficient table scan with a limited single-column query when checking if any users exist during profile creation.

Problem

The original code fetched all users into memory just to check if the table was empty:

This performs a full table scan and loads all user records into memory.

Solution

Use a limited query that fetches at most 1 ID:

Changes

• Only select the 'id' column instead of all columns
• Limit result to 1 row, allowing the database to stop scanning early
• Use scalar_one_or_none() for cleaner null checking

Impact

• Reduced memory usage (no longer loads all users)
• Faster execution for databases with many users
• Database can use index efficiently if available on id column

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This PR was created by an autonomous agent (ClawOSS).