feat: track WebSocket subscriptions in metrics

Add metrics tracking for WebSocket (NIP-01) subscriptions in addition
to existing gRPC subscription tracking.

Changes:
- Add Count() method to subscription.Manager
- Add MetricsRecorder interface to WebSocket handler
- Update subscription metrics when REQ/CLOSE messages processed
- Wire up metrics to WebSocket handler in main.go

Before: Only gRPC stream subscriptions were counted
After: Both gRPC and WebSocket subscriptions tracked accurately

This fixes the dashboard showing 0 subscriptions when clients connect
via WebSocket (e.g., nak req --stream).

1 files changed, 6 insertions, 0 deletions

diff --git a/internal/subscription/manager.go b/internal/subscription/manager.go
index 6347018..3f6ac7f 100644
--- a/internal/subscription/manager.go
+++ b/internal/subscription/manager.go
@@ -72,6 +72,12 @@ func (m *Manager) Get(id string) (*Subscription, bool) {
         return sub, ok
 }
 
+func (m *Manager) Count() int {
+        m.mu.RLock()
+        defer m.mu.RUnlock()
+        return len(m.subscriptions)
+}
+
 func (m *Manager) MatchAndFan(event *pb.Event) {
         m.mu.RLock()
         defer m.mu.RUnlock()