(feature) add Django detail views for backup runs and snapshots

Add staff-only run and snapshot detail pages so scheduler and command
output can be inspected from the Django UI.

Link dashboard and host detail tables to the new detail views, including
snapshot/base relationships and linked backup runs.

Render stored result and metadata JSON in readable form and cover the new
inspection views with tests.

src/pobsync_backend/views.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import json
+
 from django.contrib import messages
 from django.contrib.admin.views.decorators import staff_member_required
 from django.db.models import Count
@@ -100,6 +102,31 @@ def host_detail(request, host: str):
     return render(request, "pobsync_backend/host_detail.html", context)
 
 
+@staff_member_required
+def run_detail(request, run_id: int):
+    run = get_object_or_404(BackupRun.objects.select_related("host", "snapshot"), id=run_id)
+    context = {
+        "run": run,
+        "result_json": _pretty_json(run.result),
+    }
+    return render(request, "pobsync_backend/run_detail.html", context)
+
+
+@staff_member_required
+def snapshot_detail(request, snapshot_id: int):
+    snapshot = get_object_or_404(
+        SnapshotRecord.objects.select_related("host", "base").prefetch_related("derived_snapshots", "backup_runs"),
+        id=snapshot_id,
+    )
+    context = {
+        "snapshot": snapshot,
+        "metadata_json": _pretty_json(snapshot.metadata),
+        "backup_runs": snapshot.backup_runs.select_related("host").order_by("-created_at"),
+        "derived_snapshots": snapshot.derived_snapshots.select_related("host").order_by("-started_at", "dirname"),
+    }
+    return render(request, "pobsync_backend/snapshot_detail.html", context)
+
+
 @staff_member_required
 @require_POST
 def discover_host_snapshots(request, host: str):
@@ -229,3 +256,7 @@ def _default_host_initial() -> dict[str, object]:
         "retention_monthly": 12,
         "retention_yearly": 0,
     }
+
+
+def _pretty_json(value: object) -> str:
+    return json.dumps(value or {}, indent=2, sort_keys=True)