feat(reverse-proxy): clusters API surfaces type, online status, and capability flags (#6148)

The cluster listing now answers three questions in one round-trip
instead of forcing the dashboard to cross-reference the domains API:
which clusters can this account see, are they currently up, and what
do they support. The ProxyCluster wire type drops the boolean
self_hosted in favour of a `type` enum (`account` / `shared`) plus
explicit `online`, `supports_custom_ports`, `require_subdomain`, and
`supports_crowdsec` fields.

Store query reworked so offline clusters still appear (no last_seen
WHERE), with online and connected_proxies both derived from the
existing 2-min active window via portable CASE expressions; the
1-hour heartbeat reaper still removes long-stale rows. Service
manager enriches each cluster with the capability flags via the
existing per-cluster lookups (CapabilityProvider now also exposes
ClusterSupportsCrowdSec).

GetActiveClusterAddresses* keep their tight 2-min filter so service
routing and domain enumeration aren't pulled into the wider window.

The hard cut removes self_hosted from the response — the dashboard is
the only consumer and is updated in the matching PR; no transitional
field is shipped.

Adds a cross-engine regression test asserting offline clusters
surface, connected_proxies counts only fresh proxies, and
account-scoped BYOP clusters never leak across accounts.

management/internals/modules/reverseproxy/proxy/proxy.go

@@ -42,10 +42,35 @@ func (Proxy) TableName() string {
 	return "proxies"
 }
 
+// ClusterType is the source of a proxy cluster.
+type ClusterType string
+
+const (
+	// ClusterTypeAccount is a cluster operated by the account itself (BYOP) —
+	// at least one proxy row in the cluster carries a non-NULL account_id.
+	ClusterTypeAccount ClusterType = "account"
+	// ClusterTypeShared is a cluster operated by NetBird and shared across
+	// accounts — all proxy rows in the cluster have account_id IS NULL.
+	ClusterTypeShared ClusterType = "shared"
+)
+
 // Cluster represents a group of proxy nodes serving the same address.
+//
+// Online and ConnectedProxies derive from the same 2-min active window
+// the rest of the module uses, but Cluster rows are not gated on it —
+// the cluster listing surfaces offline clusters too so operators can
+// see and clean them up. The 1-hour heartbeat reaper still bounds the
+// table eventually.
 type Cluster struct {
 	ID               string
 	Address          string
+	Type             ClusterType
+	Online           bool
 	ConnectedProxies int
-	SelfHosted       bool
+	// Capability flags. *bool because nil means "no proxy reported a
+	// capability for this cluster" — the dashboard renders these as
+	// unknown rather than false.
+	SupportsCustomPorts *bool
+	RequireSubdomain    *bool
+	SupportsCrowdSec    *bool
 }