slice 8B (agent half): /backup/due cadence policy + /backup/status phases (v0.11.0)

internal/localapi: real /backup/due (cadence; due when no successful backup or
newest older than backup.backup_cadence_seconds; false in-window after success;
failed doesn't count) + /backup/status phases (idle|running|done|failed + job
id) + POST /backup single-flight with job id. Drives the controller quiesce loop.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

configs/agent.example.json

@@ -43,7 +43,8 @@
     "scratch_vmid_min": 990000,
     "scratch_vmid_max": 990009,
     "pbs_verify_cadence_seconds": 0,
-    "pbs_secret_dir": "/etc/pve/priv/storage"
+    "pbs_secret_dir": "/etc/pve/priv/storage",
+    "backup_cadence_seconds": 0
   },
   "local_api": {
     "enable": true,