feat(frontend): add backfill file hashes card to admin System tab

- System tab gains a second card with a 'Datei-Hashes berechnen' button
  that calls POST /api/admin/backfill-file-hashes and shows the updated count
- i18n: admin_system_backfill_hashes_* keys added in de/en/es
- E2E: test verifies the button triggers the backfill and shows the success message

Closes #56

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

frontend/messages/en.json

@@ -242,6 +242,10 @@
 	"admin_system_backfill_description": "Creates an initial history entry for all documents that do not have one yet (e.g. imported documents). This ensures that future edits only highlight actually changed fields.",
 	"admin_system_backfill_btn": "Backfill now",
 	"admin_system_backfill_success": "{count} documents were backfilled.",
+	"admin_system_backfill_hashes_heading": "Compute file hashes",
+	"admin_system_backfill_hashes_description": "Computes the SHA-256 hash for all previously uploaded documents that do not have one yet. This ensures annotations are correctly linked to their file version and shown again.",
+	"admin_system_backfill_hashes_btn": "Compute file hashes",
+	"admin_system_backfill_hashes_success": "{count} documents were updated.",
 	"comp_expandable_show_more": "Show more",
 	"comp_expandable_show_less": "Show less",
 	"error_comment_not_found": "The comment could not be found.",