pbs: namespace-aware client for per-customer offsite tenancy (S4)

Phase-1 live probe (felhom-hetzner) proved backup/restore/list/isolation over
the tunnel with a per-customer DatastoreBackup token, but the agent's PBS client
was namespace-unaware: Snapshots hit the datastore root (403 for a scoped token)
and Verify was whole-datastore (needs Datastore.Verify ~ admin). Operator-
approved fix.

- pbs.Config.Namespace + Client.namespace; Snapshots appends ?ns=; Verify sends
  ns= (ns-scoped verify works with DatastoreBackup on the own ns — no admin
  widening, Phase-1 confirmed). Root-ns clients unchanged (whole-datastore).
- proxmox.Storage.Namespace (parsed from /storage `namespace`).
- pbsTargetsFromPVE threads s.Namespace into the client.

Confirmed tenant ACL: DatastoreBackup on /datastore/felhom-offsite/<ns> (NOT
/ns/<ns>) to BOTH felhom@pbs (user) AND felhom@pbs!<ns> (token) — PBS privsep =
intersection; isolation holds (cross-ns 403 proven). TestClient_NamespaceScoping
red-proofed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6

internal/proxmox/types.go

@@ -218,6 +218,7 @@ type Storage struct {
 	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"
+	Namespace   string `json:"namespace,omitempty"`   // pbs namespace ("" = root; per-customer tenancy = S4)
 	VGName      string `json:"vgname,omitempty"`      // lvm/lvmthin volume group
 	ThinPool    string `json:"thinpool,omitempty"`    // lvmthin pool LV name
 }