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

internal/crypto/crypto.go

@@ -39,6 +39,9 @@ func Decrypt(key []byte, enc string) ([]byte, error) {
 }
 
 func newGCM(key []byte) (cipher.AEAD, error) {
+	if len(key) != 32 {
+		return nil, errors.New("key must be 32 bytes (AES-256)")
+	}
 	block, err := aes.NewCipher(key)
 	if err != nil {
 		return nil, err

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)")
+	}
+}