(release) Add purged snapshot audit overview

Record snapshot purge history whenever retention or incomplete cleanup removes
snapshot directories and SQL records. Store the purge reason, original kind,
path, action source, and triggering operator so manual, scheduled, CLI, and
incomplete cleanup actions remain auditable after the original snapshot record
is deleted.

Add a staff-only Purged Snapshots page with host/action filters and register
the audit model in Django admin.

Refs #16
Refs #8

src/pobsync_backend/views.py

@@ -33,7 +33,7 @@ from .forms import (
     SshCredentialForm,
 )
 from .host_ops import ensure_host_directories
-from .models import BackupRun, GlobalConfig, HostConfig, ScheduleConfig, SnapshotRecord, SshCredential
+from .models import BackupRun, GlobalConfig, HostConfig, PurgedSnapshot, ScheduleConfig, SnapshotRecord, SshCredential
 from .preflight import collect_backup_gate, effective_host_config_preview, run_remote_preflight
 from .retention import run_incomplete_cleanup, run_sql_retention_apply, run_sql_retention_plan
 from .self_check import collect_self_checks, summarize_self_checks
@@ -145,6 +145,27 @@ def logs(request):
     return render(request, "pobsync_backend/logs.html", context)
 
 
+@staff_member_required
+def purged_snapshots(request):
+    host = request.GET.get("host", "").strip()
+    action = request.GET.get("action", "").strip()
+    purged = PurgedSnapshot.objects.select_related("host").order_by("-purged_at", "host_name", "dirname")
+    if host:
+        purged = purged.filter(host_name=host)
+    if action:
+        purged = purged.filter(action=action)
+
+    context = {
+        "purged_snapshots": purged[:200],
+        "hosts": HostConfig.objects.order_by("host"),
+        "actions": PurgedSnapshot.Action.choices,
+        "selected_host": host,
+        "selected_action": action,
+        "total_count": purged.count(),
+    }
+    return render(request, "pobsync_backend/purged_snapshots.html", context)
+
+
 @staff_member_required
 def ssh_credentials(request):
     context = {
@@ -693,6 +714,8 @@ def apply_host_retention(request, host: str):
             protect_bases=protect_bases,
             yes=True,
             max_delete=form.cleaned_data["max_delete"],
+            action=PurgedSnapshot.Action.MANUAL,
+            triggered_by=request.user.get_username(),
         )
     except PobsyncError as exc:
         messages.error(request, str(exc))
@@ -733,6 +756,7 @@ def cleanup_host_incomplete_snapshots(request, host: str):
             host=host_config.host,
             yes=True,
             max_delete=form.cleaned_data["max_delete"],
+            triggered_by=request.user.get_username(),
         )
     except PobsyncError as exc:
         messages.error(request, str(exc))