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/genproto/llink/pusher/pusher.pb.go

@@ -25,6 +25,7 @@ type BulkGetPresenceRequest struct {
 	sizeCache     protoimpl.SizeCache
 	unknownFields protoimpl.UnknownFields
 
+	// Channel IDs to query. Empty means "all connected humans".
 	ChannelIds []string `protobuf:"bytes,1,rep,name=channel_ids,json=channelIds,proto3" json:"channel_ids,omitempty"`
 }
 

go/genproto/llink/pusher/pusher_grpc.pb.go

@@ -26,6 +26,9 @@ const (
 //
 // For semantics around ctx use and closing/ending streaming RPCs, please refer to https://pkg.go.dev/google.golang.org/grpc/?tab=doc#ClientConn.NewStream.
 type PusherServiceClient interface {
+	// BulkGetPresence returns presence for the given channels.
+	// When channel_ids is empty, returns all connected humanIDs across all channels
+	// under the key "_all" — useful for checking overall online status.
 	BulkGetPresence(ctx context.Context, in *BulkGetPresenceRequest, opts ...grpc.CallOption) (*BulkGetPresenceResponse, error)
 }
 
@@ -51,6 +54,9 @@ func (c *pusherServiceClient) BulkGetPresence(ctx context.Context, in *BulkGetPr
 // All implementations must embed UnimplementedPusherServiceServer
 // for forward compatibility.
 type PusherServiceServer interface {
+	// BulkGetPresence returns presence for the given channels.
+	// When channel_ids is empty, returns all connected humanIDs across all channels
+	// under the key "_all" — useful for checking overall online status.
 	BulkGetPresence(context.Context, *BulkGetPresenceRequest) (*BulkGetPresenceResponse, error)
 	mustEmbedUnimplementedPusherServiceServer()
 }