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/main/resources/db/changelog/db.changelog-master.sql

@@ -0,0 +1,28 @@
+--liquibase formatted sql
+
+--changeset editor:001-create-document
+CREATE TABLE document (
+    id          UUID PRIMARY KEY,
+    title       VARCHAR(255)             NOT NULL,
+    content     TEXT                     NOT NULL,
+    version     BIGINT                   NOT NULL,
+    created_at  TIMESTAMP WITH TIME ZONE NOT NULL,
+    updated_at  TIMESTAMP WITH TIME ZONE NOT NULL
+);
+--rollback DROP TABLE document;
+
+--changeset editor:002-create-document-history
+CREATE TABLE document_history (
+    id           BIGINT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
+    document_id  UUID                     NOT NULL,
+    version      BIGINT                   NOT NULL,
+    title        VARCHAR(255)             NOT NULL,
+    content      TEXT                     NOT NULL,
+    change_type  VARCHAR(16)              NOT NULL,
+    change_time  TIMESTAMP WITH TIME ZONE NOT NULL
+);
+--rollback DROP TABLE document_history;
+
+--changeset editor:003-index-document-history
+CREATE INDEX idx_document_history_document_id ON document_history (document_id);
+--rollback DROP INDEX idx_document_history_document_id;