(feature) Show backup data totals by snapshot kind

Aggregate snapshot storage metadata by snapshot kind so operators can see
scheduled, manual, incomplete, and total backup data.

Surface the totals per host and across all hosts on the dashboard, using
allocated snapshot size from recorded backup metadata without rescanning
backup storage.

src/pobsync_backend/tests/test_stats_summary.py

@@ -0,0 +1,64 @@
+from __future__ import annotations
+
+from datetime import datetime, timezone
+
+from django.test import TestCase
+
+from pobsync_backend.models import HostConfig, SnapshotRecord
+from pobsync_backend.stats_summary import collect_dashboard_stats, collect_host_stats
+
+
+class StatsSummaryTests(TestCase):
+    def test_collect_dashboard_stats_sums_backup_data_across_hosts(self) -> None:
+        web = HostConfig.objects.create(host="web-01", address="web-01.example.test")
+        db = HostConfig.objects.create(host="db-01", address="db-01.example.test")
+        self._snapshot(web, "20260519-021500Z__SCHED01", SnapshotRecord.Kind.SCHEDULED, allocated=100)
+        self._snapshot(web, "20260519-031500Z__MANUAL1", SnapshotRecord.Kind.MANUAL, allocated=200)
+        self._snapshot(db, "20260519-041500Z__SCHED02", SnapshotRecord.Kind.SCHEDULED, allocated=300)
+        self._snapshot(db, "20260519-051500Z__BROKEN1", SnapshotRecord.Kind.INCOMPLETE, allocated=400)
+
+        stats = collect_dashboard_stats(hosts=[web, db], global_config=None)
+
+        self.assertEqual(stats["backup_data"]["scheduled"]["count"], 2)
+        self.assertEqual(stats["backup_data"]["scheduled"]["allocated_size_bytes"], 400)
+        self.assertEqual(stats["backup_data"]["manual"]["allocated_size_bytes"], 200)
+        self.assertEqual(stats["backup_data"]["incomplete"]["allocated_size_bytes"], 400)
+        self.assertEqual(stats["backup_data"]["total"]["count"], 4)
+        self.assertEqual(stats["backup_data"]["total"]["allocated_size_bytes"], 1000)
+
+    def test_collect_host_stats_sums_backup_data_by_snapshot_kind(self) -> None:
+        host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
+        self._snapshot(host, "20260519-021500Z__SCHED01", SnapshotRecord.Kind.SCHEDULED, allocated=100)
+        self._snapshot(host, "20260519-031500Z__SCHED02", SnapshotRecord.Kind.SCHEDULED, allocated=200)
+        self._snapshot(host, "20260519-041500Z__MANUAL1", SnapshotRecord.Kind.MANUAL, allocated=300)
+        self._snapshot(host, "20260519-051500Z__BROKEN1", SnapshotRecord.Kind.INCOMPLETE, allocated=400)
+
+        stats = collect_host_stats(host=host)
+
+        self.assertEqual(stats["backup_data"]["scheduled"]["count"], 2)
+        self.assertEqual(stats["backup_data"]["scheduled"]["allocated_size_bytes"], 300)
+        self.assertEqual(stats["backup_data"]["manual"]["allocated_size_bytes"], 300)
+        self.assertEqual(stats["backup_data"]["incomplete"]["allocated_size_bytes"], 400)
+        self.assertEqual(stats["backup_data"]["total"]["count"], 4)
+        self.assertEqual(stats["backup_data"]["total"]["allocated_size_bytes"], 1000)
+
+    def _snapshot(self, host: HostConfig, dirname: str, kind: str, *, allocated: int) -> SnapshotRecord:
+        started_at = datetime.strptime(dirname.split("__", 1)[0], "%Y%m%d-%H%M%SZ").replace(tzinfo=timezone.utc)
+        return SnapshotRecord.objects.create(
+            host=host,
+            kind=kind,
+            dirname=dirname,
+            path=f"/backups/{host.host}/{kind}/{dirname}",
+            status="success",
+            started_at=started_at,
+            metadata={
+                "stats": {
+                    "storage": {
+                        "snapshot": {
+                            "apparent_size_bytes": allocated * 2,
+                            "allocated_size_bytes": allocated,
+                        }
+                    }
+                }
+            },
+        )