hub: accept controller agent_channel_* events (channel-health relay) v0.20.0

The controller v0.90.0 channel-health checker pushes agent_channel_* events to /api/v1/event;
the allowedEventTypes allowlist rejected them (HTTP 400). Added the 8 types (operator-only).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EPZ4GJ8L5Jqf8UiPwbn1kt

hub/internal/api/handler.go

@@ -1078,6 +1078,15 @@ var allowedEventTypes = map[string]bool{
 	"app_removed":                 true,
 	"disaster_recovery_started":   true,
 	"disaster_recovery_completed": true,
+	// Controller→agent channel health (controller v0.90.0) — operator-only (not customer toggles)
+	"agent_channel_pin_mismatch":       true,
+	"agent_channel_unauthorized":       true,
+	"agent_channel_unreachable":        true,
+	"agent_channel_timeout":            true,
+	"agent_channel_misconfigured":      true,
+	"agent_channel_construction_error": true,
+	"agent_channel_unknown":            true,
+	"agent_channel_recovered":          true,
 	// Hub-generated events
 	"node_stale":     true,
 	"node_down":      true,