Health: learn n_ctx and total_slots from /props

Implemented /props learning in internal/health/health.go (added Status.NCtx/Status.Slots, PerSlotCtx, and a best-effort GET <base>/props appended to the poll after /v1/models; 0/unknown on any failure without failing the poll) and exposed them in internal/admin/admin.go HostView. Copied internal/health/props_test.go and the replacement internal/proxy/helpers_test.go byte-identical to docs/plans/v2/_files/.

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

internal/admin/admin.go

@@ -38,6 +38,8 @@ type HostView struct {
 	InFlight  int      `json:"in_flight"`  // sum over the host's configured models
 	Queued    int      `json:"queued"`     // same
 	Draining  bool     `json:"draining"`
+	NCtx      int      `json:"n_ctx"` // from /props; 0 = unknown
+	Slots     int      `json:"slots"` // from /props; 0 = unknown
 }
 
 // LeaseView is one lease's row in a route's leases.
@@ -125,6 +127,8 @@ func (hx *handler) hostView(name string, s health.Status) HostView {
 		InFlight:  inflight,
 		Queued:    queued,
 		Draining:  hx.d.Draining(name),
+		NCtx:      s.NCtx,
+		Slots:     s.Slots,
 	}
 }