fix(auth): handle unique-constraint violation in register as 409 Conflict (#55)

[ghzhost]

Overview

Closes #55.

When two concurrent POST /api/auth/register requests race on the same email, both could pass the initial SELECT EXISTS(...) query, hash the password via bcrypt (~100-300ms race window), and attempt INSERT INTO users. The database UNIQUE constraint correctly prevents duplication, but the second insert previously triggered a 500 AppError::Database / internal server error rather than 409 Conflict.

Changes

1. Promoted is_unique_violation(error: &sqlx::Error) -> bool to crate::db::is_unique_violation as a shared helper across the codebase.
2. Updated services::batch, services::escrow, and services::subscription to use crate::db::is_unique_violation rather than duplicating the helper function.
3. Wrapped register's INSERT INTO users query with .map_err(...) checking is_unique_violation(&e) and mapping unique constraint violations directly to AppError::Conflict("Email".into()).
4. Added unit and concurrency database test coverage asserting racing registrations properly return 409 Conflict instead of 500 error.