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/shared/grpc/validate_session_test.go

@@ -322,7 +322,7 @@ func (m *testValidateSessionServiceManager) GetServiceByDomain(ctx context.Conte
 	return m.store.GetServiceByDomain(ctx, domain)
 }
 
-func (m *testValidateSessionServiceManager) GetActiveClusters(_ context.Context, _, _ string) ([]proxy.Cluster, error) {
+func (m *testValidateSessionServiceManager) GetClusters(_ context.Context, _, _ string) ([]proxy.Cluster, error) {
 	return nil, nil
 }