Fix ESPI compliance: Remove ObjectEntity and make all customer entities ESPI compliant

ARCHITECTURE FIX:
- Remove ObjectEntity class (violated ESPI standards)
- Migrate AccountNotificationEntity and StatementRefEntity to extend IdentifiedObjectEntity
- Ensure ALL customer entities are ESPI compliant with proper UUID identifiers

BENEFITS:
- Consistent architecture across usage and customer domains
- ESPI compliant UUID5 identifiers for all entities
- ATOM feed compatibility for customer resources
- Proper href generation and link management
- Timestamp management and audit trails

COMPLIANCE:
- All entities now follow NAESB ESPI 1.0 specification
- Removes architectural inconsistency between domains
- Enables proper Green Button interoperability for customer data

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: dfcoffin

src/main/java/org/greenbuttonalliance/espi/common/domain/customer/entity/AccountNotificationEntity.java

@@ -25,6 +25,7 @@
 import lombok.NoArgsConstructor;
 import lombok.ToString;
 import org.greenbuttonalliance.espi.common.domain.customer.enums.NotificationMethodKind;
+import org.greenbuttonalliance.espi.common.domain.usage.IdentifiedObjectEntity;
 
 import jakarta.persistence.*;
 import java.time.OffsetDateTime;
@@ -33,14 +34,17 @@
  * Pure JPA/Hibernate entity for AccountNotification without JAXB concerns.
  * 
  * [extension] Customer action notification (e.g., delinquency, move in, move out)
+ * ESPI compliant with proper UUID identifiers and ATOM feed support.
  */
 @Entity
-@Table(name = "account_notifications")
+@Table(name = "account_notifications", uniqueConstraints = {
+    @UniqueConstraint(columnNames = {"uuid"})
+})
 @Data
 @EqualsAndHashCode(callSuper = true)
 @NoArgsConstructor
 @ToString(callSuper = true)
-public class AccountNotificationEntity extends ObjectEntity {
+public class AccountNotificationEntity extends IdentifiedObjectEntity {
 
     /**
      * Method by which the customer was notified.

src/main/java/org/greenbuttonalliance/espi/common/domain/customer/entity/ObjectEntity.java

@@ -24,21 +24,25 @@
 import lombok.EqualsAndHashCode;
 import lombok.NoArgsConstructor;
 import lombok.ToString;
+import org.greenbuttonalliance.espi.common.domain.usage.IdentifiedObjectEntity;
 
 import jakarta.persistence.*;
 
 /**
  * Pure JPA/Hibernate entity for StatementRef without JAXB concerns.
  * 
  * [extension] A sequence of references to a document associated with a Statement.
+ * ESPI compliant with proper UUID identifiers and ATOM feed support.
  */
 @Entity
-@Table(name = "statement_refs")
+@Table(name = "statement_refs", uniqueConstraints = {
+    @UniqueConstraint(columnNames = {"uuid"})
+})
 @Data
 @EqualsAndHashCode(callSuper = true)
 @NoArgsConstructor
 @ToString(callSuper = true)
-public class StatementRefEntity extends ObjectEntity {
+public class StatementRefEntity extends IdentifiedObjectEntity {
 
     /**
      * [extension] Name of document or file including filename extension if present.