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_en.json

@@ -460,5 +460,25 @@
   "Progress": "Progress",
   "Created by": "Created by",
   "Search handovers...": "Search handovers...",
-  "ID": "ID"
+  "ID": "ID",
+  "History": "History",
+  "Current": "Current",
+  "Viewing version %d": "Viewing version %d",
+  "compared with version %d": "compared with version %d",
+  "Back to current": "Back to current",
+  "Restore this version": "Restore this version",
+  "Restore this version?": "Restore this version?",
+  "Version restored": "Version restored",
+  "Only managers can restore revisions": "Only managers can restore revisions",
+  "Revision not found": "Revision not found",
+  "Created": "Created",
+  "Fields changed": "Fields changed",
+  "Status changed": "Status changed",
+  "Fields and status changed": "Fields and status changed",
+  "Restored": "Restored",
+  "was: %s": "was: %s",
+  "Yes": "Yes",
+  "No": "No",
+  "Changed": "Changed",
+  "Compare with current": "Compare with current"
 }