fix: tighten password and surface persistence errors

internal/service/auth_password_internal_test.go

@@ -0,0 +1,12 @@
+package service
+
+import "testing"
+
+func TestValidatePasswordStrengthBoundaryRules(t *testing.T) {
+	t.Run("accepts boundary password with three character classes", func(t *testing.T) {
+		err := validatePasswordStrength("Abcd1234", defaultPasswordMinLen, false)
+		if err != nil {
+			t.Fatalf("expected 8-char password with three classes to pass: %v", err)
+		}
+	})
+}