feat: add DocumentVersion entity, repository, service, and migration

Creates the document_versions table (V9) with JSONB snapshot and
changed_fields columns. DocumentVersionService records a version on
every create/update, resolves the editor name from the security context,
and computes changedFields by diffing against the previous snapshot.

Refs #38
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V9__add_document_versions.sql

@@ -0,0 +1,11 @@
+CREATE TABLE document_versions (
+    id            UUID         PRIMARY KEY DEFAULT gen_random_uuid(),
+    document_id   UUID         NOT NULL REFERENCES documents(id) ON DELETE CASCADE,
+    saved_at      TIMESTAMP    NOT NULL DEFAULT now(),
+    editor_id     UUID         REFERENCES users(id) ON DELETE SET NULL,
+    editor_name   VARCHAR(200) NOT NULL,
+    snapshot      JSONB        NOT NULL,
+    changed_fields JSONB       NOT NULL DEFAULT '[]'
+);
+
+CREATE INDEX ON document_versions (document_id, saved_at DESC);