fix(crypto): enforce 32-byte key for AES-256

internal/crypto/crypto_test.go

@@ -31,3 +31,9 @@ func TestEncryptNonDeterministic(t *testing.T) {
 		t.Fatal("two encryptions must differ (random nonce)")
 	}
 }
+
+func TestEncryptRejectsWrongKeySize(t *testing.T) {
+	if _, err := Encrypt(make([]byte, 16), []byte("x")); err == nil {
+		t.Fatal("16-byte key must be rejected (AES-256 requires 32)")
+	}
+}