(refactor) make Docker backup root static in Django setup

Remove backup_root from the normal Django global config form and display
the fixed container path /backups instead.

Always persist /backups from the setup form so Docker deployments do not
mix host paths with container paths.

Update tests and docs to clarify that the host backup directory is chosen
through the Docker mount, while Django always uses /backups internally.

src/pobsync_backend/views.py

@@ -4,6 +4,7 @@ import json
 
 from django.contrib import messages
 from django.contrib.admin.views.decorators import staff_member_required
+from django.conf import settings
 from django.db.models import Count
 from django.shortcuts import get_object_or_404, redirect, render
 from django.views.decorators.http import require_POST
@@ -60,6 +61,7 @@ def edit_global_config(request):
         {
             "global_config": global_config,
             "form": form,
+            "backup_root": settings.POBSYNC_BACKUP_ROOT,
         },
     )
 
@@ -264,7 +266,6 @@ def _default_schedule_initial() -> dict[str, object]:
 def _default_global_initial() -> dict[str, object]:
     return {
         "name": "default",
-        "backup_root": "/backups",
         "ssh_user": "root",
         "ssh_port": 22,
         "rsync_binary": "rsync",