fix(production): JPA-Save-Pattern, Optimistic Locking und Domain-Validierung

- Managed-Entity-Update statt detach/merge (verhindert DELETE+INSERT-Churn)
- @Version für Optimistic Locking mit ConcurrentModification-Error
- Null-Checks für quantityUsed/quantityUnit vor BigDecimal-Parsing
- Duplicate-Check nach Consumption.create() für robustere Validierung
- FetchType.EAGER→LAZY für BatchEntity.consumptions
- Liquibase-Migration 020 für version-Spalte

backend/src/main/resources/db/changelog/changes/020-add-version-to-batches.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<databaseChangeLog
+        xmlns="http://www.liquibase.org/xml/ns/dbchangelog"
+        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+        xsi:schemaLocation="http://www.liquibase.org/xml/ns/dbchangelog
+        http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-latest.xsd">
+
+    <changeSet id="020-add-version-to-batches" author="effigenix">
+        <addColumn tableName="batches">
+            <column name="version" type="bigint" defaultValueNumeric="0">
+                <constraints nullable="false"/>
+            </column>
+        </addColumn>
+    </changeSet>
+
+</databaseChangeLog>

backend/src/main/resources/db/changelog/db.changelog-master.xml

@@ -24,5 +24,6 @@
     <include file="db/changelog/changes/017-timestamps-to-timestamptz.xml"/>
     <include file="db/changelog/changes/018-add-article-id-to-recipes.xml"/>
     <include file="db/changelog/changes/019-create-batch-consumptions-table.xml"/>
+    <include file="db/changelog/changes/020-add-version-to-batches.xml"/>
 
 </databaseChangeLog>