Remove authentication level 1 SQL injection

[r1kka6]

Summary

• replace the Level 1 string-concatenated SQL query with the existing repository-backed authentication flow
• remove the unused JdbcTemplate path from AuthLoginService
• update the unit test to verify the repository lookup contract instead of mocked raw SQL execution

Verification

• ./gradlew compileJava test spotlessJavaCheck bootJar --no-daemon passed in an eclipse-temurin:17-jdk container
• vulnerable baseline authenticates username=' OR 1=1 -- with any password\n- patched runtime rejects that payload while admin_sqli with not_needed_for_sqli still succeeds and wrong passwords still fail