(feature) improve snapshot discovery visibility in Django

Add a discovery preflight that reports the configured backup root, host
root, and snapshot directory counts before importing anything.

Show discovery status on host detail pages so missing mounts or mismatched
host directories are visible from the UI.

Warn clearly when discovery scans zero snapshots, including whether the
host backup directory is missing or simply empty.

src/pobsync_backend/views.py

@@ -15,7 +15,7 @@ from .backup_runner import queue_backup_run
 from .forms import CreateHostConfigForm, GlobalConfigForm, HostConfigForm, ManualBackupForm, ScheduleConfigForm
 from .models import BackupRun, GlobalConfig, HostConfig, ScheduleConfig, SnapshotRecord
 from .retention import run_sql_retention_plan
-from .snapshot_discovery import discover_snapshots
+from .snapshot_discovery import discover_snapshots, inspect_snapshot_discovery
 
 
 @staff_member_required
@@ -93,6 +93,7 @@ def host_detail(request, host: str):
     context = {
         "host": host_config,
         "schedule": _schedule_for_host(host_config),
+        "discovery": inspect_snapshot_discovery(host=host_config),
         "manual_backup_form": ManualBackupForm(initial=_default_manual_backup_initial(host_config)),
         "latest_runs": host_config.runs.select_related("snapshot").order_by("-created_at")[:10],
         "snapshots": host_config.snapshots.select_related("base").order_by("-started_at", "dirname")[:20],
@@ -167,13 +168,15 @@ def discover_host_snapshots(request, host: str):
     except Exception as exc:
         messages.error(request, f"Snapshot discovery failed for {host_config.host}: {exc}")
     else:
-        messages.success(
-            request,
-            (
-                f"Snapshot discovery scanned {result['scanned']} items for {host_config.host}: "
-                f"{result['created']} created, {result['updated']} updated."
-            ),
+        summary = (
+            f"Snapshot discovery scanned {result['scanned']} items for {host_config.host}: "
+            f"{result['created']} created, {result['updated']} updated."
         )
+        if result["scanned"]:
+            messages.success(request, summary)
+        else:
+            discovery = inspect_snapshot_discovery(host=host_config)
+            messages.warning(request, f"{summary} {discovery['message']}")
     return redirect("host_detail", host=host_config.host)