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

cmd/felhom-agent/main.go

@@ -753,7 +753,7 @@ func pbsTargetsFromPVE(cfg config.Config, px *proxmox.Client, logger *slog.Logge
 				logger.Warn("pbs: cannot read token secret; skipping datastore", "storage", s.Storage, "err", err)
 				continue
 			}
-			c, err := pbs.NewClient(pbs.Config{Server: s.Server, Fingerprint: s.Fingerprint, TokenID: s.Username, Secret: secret})
+			c, err := pbs.NewClient(pbs.Config{Server: s.Server, Fingerprint: s.Fingerprint, TokenID: s.Username, Secret: secret, Namespace: s.Namespace})
 			if err != nil {
 				logger.Warn("pbs: cannot build client; skipping datastore", "storage", s.Storage, "err", err)
 				continue