hub v0.7.3: ingest agent backups + restore_tests (slice 6 Phase A)

Accept + persist the now-populated host-report backups/restore_tests. Mirror structs in
hostReportPayload; persisted via report_json (no schema change); a FAILED restore-test is
logged prominently (loudest DR signal). Shared golden updated byte-identical with
felhom-agent; bidirectional key-set tests added. Build/deploy deferred (backward-compatible).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

hub/internal/api/testdata/host-report.golden.json

@@ -86,8 +86,31 @@
       }
     }
   ],
-  "backups": [],
-  "restore_tests": [],
+  "backups": [
+    {
+      "target_id": "local",
+      "vmid": 9001,
+      "archive": "local:backup/vzdump-lxc-9001-2026_06_09-11_00_00.tar.zst",
+      "mode": "snapshot",
+      "crash_consistent": true,
+      "size_bytes": 524288000,
+      "success": true,
+      "started_at": "2026-06-09T11:00:00Z",
+      "duration_seconds": 42.5,
+      "uncovered_volumes": ["/mnt/bulk"]
+    }
+  ],
+  "restore_tests": [
+    {
+      "source_archive": "local:backup/vzdump-lxc-9001-2026_06_09-11_00_00.tar.zst",
+      "source_tier": "local",
+      "scratch_vmid": 990000,
+      "pass": true,
+      "verified": "boot+running",
+      "tested_at": "2026-06-09T11:05:00Z",
+      "duration_seconds": 38.2
+    }
+  ],
   "pbs_snapshots": [],
   "cloudflared": { "status": "active" },
   "audit_tail": []