feat(helpdesk): add handover revision history with timeline, diff, and restore

Every save creates an immutable revision snapshot. The aside shows a
commit-graph-style timeline (newest first) with vertical line, circle
nodes, version numbers, change type, user, and date. Clicking a past
revision renders it readonly with inline git-diff-style highlights
(changed/added/removed with tinted backgrounds). Compare mode allows
diffing any two arbitrary revisions. MANAGE users can restore old
versions, which creates a new revision preserving full history.

New: HandoverRevisionService, HandoverRevisionRepository,
migration 006, 14 PHPUnit tests, DE/EN translations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

modules/helpdesk/i18n/default_de.json

@@ -460,5 +460,25 @@
   "Progress": "Fortschritt",
   "Created by": "Erstellt von",
   "Search handovers...": "Übergaben suchen...",
-  "ID": "ID"
+  "ID": "ID",
+  "History": "Verlauf",
+  "Current": "Aktuell",
+  "Viewing version %d": "Version %d wird angezeigt",
+  "compared with version %d": "verglichen mit Version %d",
+  "Back to current": "Zurück zur aktuellen Version",
+  "Restore this version": "Diese Version wiederherstellen",
+  "Restore this version?": "Diese Version wiederherstellen?",
+  "Version restored": "Version wiederhergestellt",
+  "Only managers can restore revisions": "Nur Manager können Versionen wiederherstellen",
+  "Revision not found": "Version nicht gefunden",
+  "Created": "Erstellt",
+  "Fields changed": "Felder geändert",
+  "Status changed": "Status geändert",
+  "Fields and status changed": "Felder und Status geändert",
+  "Restored": "Wiederhergestellt",
+  "was: %s": "war: %s",
+  "Yes": "Ja",
+  "No": "Nein",
+  "Changed": "Geändert",
+  "Compare with current": "Mit aktueller Version vergleichen"
 }