(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/tests/test_views.py

@@ -218,10 +218,28 @@ class ViewTests(TestCase):
         self.assertContains(response, "Edit schedule")
         self.assertContains(response, "Edit config")
         self.assertContains(response, "Queue Manual Backup")
+        self.assertContains(response, "Snapshot Discovery")
         self.assertContains(response, reverse("queue_manual_backup", args=[host.host]))
         self.assertContains(response, reverse("run_detail", args=[BackupRun.objects.get().id]))
         self.assertContains(response, reverse("snapshot_detail", args=[snapshot.id]))
 
+    def test_host_detail_renders_discovery_status_for_existing_snapshot_dirs(self) -> None:
+        self.client.force_login(self.staff_user)
+        with TemporaryDirectory() as tmp:
+            backup_root = Path(tmp) / "backups"
+            GlobalConfig.objects.create(name="default", backup_root=str(backup_root))
+            host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
+            (backup_root / host.host / "scheduled" / "20260519-021500Z__ABCDEFGH").mkdir(parents=True)
+            (backup_root / host.host / ".incomplete" / "20260519-031500Z__BROKEN01").mkdir(parents=True)
+
+            response = self.client.get(reverse("host_detail", args=[host.host]))
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(response, f"Host root:</strong> {backup_root / host.host}")
+        self.assertContains(response, "Found 2 snapshot directories")
+        self.assertContains(response, "scheduled 1")
+        self.assertContains(response, "incomplete 1")
+
     def test_host_detail_returns_404_for_unknown_host(self) -> None:
         self.client.force_login(self.staff_user)
 
@@ -352,6 +370,20 @@ class ViewTests(TestCase):
         self.assertContains(response, "Snapshot discovery scanned 1 items")
         self.assertTrue(SnapshotRecord.objects.filter(host=host, dirname=snapshot_dir.name).exists())
 
+    def test_discover_host_snapshots_warns_when_host_root_is_missing(self) -> None:
+        self.client.force_login(self.staff_user)
+        with TemporaryDirectory() as tmp:
+            backup_root = Path(tmp) / "backups"
+            GlobalConfig.objects.create(name="default", backup_root=str(backup_root))
+            host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
+
+            response = self.client.post(reverse("discover_host_snapshots", args=[host.host]), follow=True)
+
+        self.assertRedirects(response, reverse("host_detail", args=[host.host]))
+        self.assertContains(response, "Snapshot discovery scanned 0 items")
+        self.assertContains(response, "Host backup directory does not exist yet")
+        self.assertFalse(SnapshotRecord.objects.exists())
+
     def test_discover_host_snapshots_requires_post(self) -> None:
         self.client.force_login(self.staff_user)
         host = HostConfig.objects.create(host="web-01", address="web-01.example.test")