feat: send email notifications for missed messages

- New cron job in cluster
- Handle presence and other concerns
- Toggle in app to disable email notifications
- Handles other edge cases such as cooldown period
- Simple html email with simple message

Closes #117

go/internal/pusher/server.go

@@ -72,7 +72,22 @@ func (s *Server) HandleWebSocket(w http.ResponseWriter, r *http.Request) {
 }
 
 // BulkGetPresence implements the gRPC PusherService.
+// When channel_ids is empty, returns all connected humanIDs across all channels
+// under the key "_all" — useful for checking overall online status.
 func (s *Server) BulkGetPresence(ctx context.Context, req *pbpusher.BulkGetPresenceRequest) (*pbpusher.BulkGetPresenceResponse, error) {
+	// Empty channel_ids → return all connected humans
+	if len(req.ChannelIds) == 0 {
+		humanIDs, err := s.bridge.GetAllConnectedHumanIDs(ctx)
+		if err != nil {
+			return nil, err
+		}
+		return &pbpusher.BulkGetPresenceResponse{
+			Presences: map[string]*pbpusher.ChannelPresence{
+				"_all": {HumanIds: humanIDs},
+			},
+		}, nil
+	}
+
 	presence, err := s.bridge.GetPresence(ctx, req.ChannelIds)
 	if err != nil {
 		return nil, err