refactor: apply clippy let-chains and harden password validation

- Auto-fix 29 clippy warnings by converting nested if-lets to let-chains
- Harden password strength validator to reject restricted substrings
- Simplify rate_limiter state checks using is_none_or
- Improves idiomatic Rust style and overall security posture

Cargo.lock

@@ -1239,7 +1239,7 @@ dependencies = [
 
 [[package]]
 name = "nx9-auth"
-version = "0.3.0"
+version = "0.4.0"
 dependencies = [
  "anyhow",
  "argon2",