initial backend: Dokumente, Historie, API und Persistenz (D76)

- Liquibase-Schema (`document`, `document_history`) + Rollback-scripts
- Spring Boot with JPA-repositories, entities and services
- REST-API (`/documents`, `/documents/{id}`, `/documents/{id}/history`)
- OpenAPI-specifikation for CRUD-Operationen and history
- config files (`application.yaml`, `Liquibase`, H2 im PostgreSQL-Modus)
- preps for future live-editing/delta-updates
- Exceptions for conflikt- and not-found cases (409/404)
- keeping document hostory even after `delete` for RESTORE functionality

backend/src/test/resources/application.yaml

@@ -0,0 +1,12 @@
+spring:
+  datasource:
+    url: jdbc:h2:mem:editor-test;MODE=PostgreSQL;DATABASE_TO_LOWER=TRUE;DEFAULT_NULL_ORDERING=HIGH;DB_CLOSE_DELAY=-1
+    username: sa
+    password: ""
+    driver-class-name: org.h2.Driver
+  jpa:
+    hibernate:
+      ddl-auto: validate
+    open-in-view: false
+  liquibase:
+    change-log: classpath:db/changelog/db.changelog-master.sql