feat: add Subscription enum, converter, entity lifecycle hooks, and ORM-only test rule

CODING_GUIDELINES.md

@@ -293,6 +293,10 @@ public class GlobalExceptionHandler {
 - Frontend: Vitest for composables and utility functions. Cypress or Playwright for E2E (Phase 1).
 - Test naming: `shouldXxxWhenYyy` — e.g., `shouldReturn404WhenPlateNotFound`.
 - Aim for test coverage on business logic, not on getters/setters/boilerplate.
+- All database interaction in tests must go through JPA repositories
+  or EntityManager. Never use JdbcTemplate, DataSource queries, or
+  raw SQL in test code. Tests interact with the database the same way
+  production code does: through the ORM.
 
 ---
 

backend/src/main/java/se/bilhalsning/entity/SubscriptionConverter.java

@@ -0,0 +1,26 @@
+package se.bilhalsning.entity;
+
+import jakarta.persistence.AttributeConverter;
+import jakarta.persistence.Converter;
+
+@Converter(autoApply = true)
+public class SubscriptionConverter implements AttributeConverter<Subscription, String> {
+
+    @Override
+    public String convertToDatabaseColumn(Subscription subscription) {
+        return subscription != null ? subscription.getValue() : null;
+    }
+
+    @Override
+    public Subscription convertToEntityAttribute(String dbData) {
+        if (dbData == null) {
+            return null;
+        }
+        for (Subscription subscription : Subscription.values()) {
+            if (subscription.getValue().equals(dbData)) {
+                return subscription;
+            }
+        }
+        throw new IllegalArgumentException("Unknown subscription value: " + dbData);
+    }
+}

backend/src/main/java/se/bilhalsning/entity/User.java

@@ -2,8 +2,7 @@ package se.bilhalsning.entity;
 
 import jakarta.persistence.Column;
 import jakarta.persistence.Entity;
-import jakarta.persistence.EnumType;
-import jakarta.persistence.Enumerated;
+import jakarta.persistence.Convert;
 import jakarta.persistence.Id;
 import jakarta.persistence.PrePersist;
 import jakarta.persistence.PreUpdate;
@@ -25,7 +24,7 @@ public class User {
     @Column(name = "password_hash", nullable = false, length = 255)
     private String passwordHash;
 
-    @Enumerated(EnumType.STRING)
+    @Convert(converter = SubscriptionConverter.class)
     @Column(name = "subscription", nullable = false, length = 20)
     private Subscription subscription = Subscription.NONE;