test: add OrderStatusConverter and SubscriptionConverter unit tests

- OrderStatusConverterTest (6 tests): null-to-null, value-to-string,
  string-to-enum matching, null-to-null reverse, invalid string throws
  IllegalArgumentException, roundtrip all 6 OrderStatus values
- SubscriptionConverterTest (6 tests): same pattern for 3 subscription
  values (NONE/BASIC/PRO)
- Pure unit tests — no Spring context, no database
- Raises backend branch coverage from 45.5% to 77.3% (both converters
  now at 100% branch and line coverage)
- Unblocks ./gradlew check: the 60% branch threshold was previously
  failing due to untested converter logic

backend/src/test/java/se/bilhalsning/entity/OrderStatusConverterTest.java

@@ -0,0 +1,46 @@
+package se.bilhalsning.entity;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+class OrderStatusConverterTest {
+
+    private final OrderStatusConverter converter = new OrderStatusConverter();
+
+    @Test
+    void shouldReturnValueWhenStatusIsNotNull() {
+        assertEquals("sent", converter.convertToDatabaseColumn(OrderStatus.SENT));
+    }
+
+    @Test
+    void shouldReturnNullWhenStatusIsNull() {
+        assertNull(converter.convertToDatabaseColumn(null));
+    }
+
+    @Test
+    void shouldReturnEnumWhenDbDataMatchesValue() {
+        assertEquals(OrderStatus.PAID, converter.convertToEntityAttribute("paid"));
+    }
+
+    @Test
+    void shouldReturnNullWhenDbDataIsNull() {
+        assertNull(converter.convertToEntityAttribute(null));
+    }
+
+    @Test
+    void shouldThrowWhenDbDataDoesNotMatchAnyStatus() {
+        assertThrows(IllegalArgumentException.class,
+                () -> converter.convertToEntityAttribute("bogus"));
+    }
+
+    @Test
+    void shouldRoundtripAllEnumValues() {
+        for (OrderStatus status : OrderStatus.values()) {
+            String db = converter.convertToDatabaseColumn(status);
+            assertEquals(status, converter.convertToEntityAttribute(db));
+        }
+    }
+}

backend/src/test/java/se/bilhalsning/entity/SubscriptionConverterTest.java

@@ -0,0 +1,46 @@
+package se.bilhalsning.entity;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+class SubscriptionConverterTest {
+
+    private final SubscriptionConverter converter = new SubscriptionConverter();
+
+    @Test
+    void shouldReturnValueWhenSubscriptionIsNotNull() {
+        assertEquals("basic", converter.convertToDatabaseColumn(Subscription.BASIC));
+    }
+
+    @Test
+    void shouldReturnNullWhenSubscriptionIsNull() {
+        assertNull(converter.convertToDatabaseColumn(null));
+    }
+
+    @Test
+    void shouldReturnEnumWhenDbDataMatchesValue() {
+        assertEquals(Subscription.PRO, converter.convertToEntityAttribute("pro"));
+    }
+
+    @Test
+    void shouldReturnNullWhenDbDataIsNull() {
+        assertNull(converter.convertToEntityAttribute(null));
+    }
+
+    @Test
+    void shouldThrowWhenDbDataDoesNotMatchAnySubscription() {
+        assertThrows(IllegalArgumentException.class,
+                () -> converter.convertToEntityAttribute("premium"));
+    }
+
+    @Test
+    void shouldRoundtripAllEnumValues() {
+        for (Subscription subscription : Subscription.values()) {
+            String db = converter.convertToDatabaseColumn(subscription);
+            assertEquals(subscription, converter.convertToEntityAttribute(db));
+        }
+    }
+}