v0.6.0: slice 6 Phase B — PBS offsite tier (verify + PBS-API client + reporting)

Spike-proven that backup/restore-to-PBS reuse Phase A unchanged; the only new code is
the verify capability, a small PBS-API client, and PBSSnapshot reporting.

- internal/pbs: fingerprint-pinned, token-authed PBS-API client (Verify/Snapshots/
  TaskStatus, node-from-UPID; secret read from /etc/pve/priv/storage/<id>.pw at runtime,
  never logged) + the verify maintenance loop (own cadence, default 6h, NOT gated/journaled,
  like the watchdog) + SnapshotStore.
- hub: PBSSnapshot filled (namespace/type/id/time/size/owner/protected/encrypted/
  verify_state/verify_upid); PBSReporter collector seam; cross-repo golden + bidirectional
  key-set tests; hub handler parses pbs_snapshots + logs a failed-verify WARN.
- backup: report the ACTUAL vzdump mode (parsed from the task log; PVE may downgrade
  snapshot->stop). proxmox.Storage.Username. config PBSVerifyCadence/secret-dir.
  --selftest=pbs-verify. Backup/restore-to-PBS unchanged.

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

internal/proxmox/types.go

@@ -163,6 +163,7 @@ type Storage struct {
 	Share       string `json:"share,omitempty"`       // cifs share name
 	Datastore   string `json:"datastore,omitempty"`   // pbs datastore name
 	Fingerprint string `json:"fingerprint,omitempty"` // pbs server cert fingerprint
+	Username    string `json:"username,omitempty"`    // pbs auth id, e.g. "felhom@pbs!n100"
 	VGName      string `json:"vgname,omitempty"`      // lvm/lvmthin volume group
 	ThinPool    string `json:"thinpool,omitempty"`    // lvmthin pool LV name
 }