fix: Bitget WS keepalive — send text ping, not WebSocket PingMessage

Bitget v2 WS requires a text message "ping" every 30s, not a WebSocket
PingMessage control frame (opcode 0x9). Using the wrong ping type caused
a silent failure: connection stays up and subscription succeeds, but NO
ticker data is pushed — zero errors, zero reconnection logs.

Changes:
- connector.go: Add TextPing bool flag, send text 'ping' when set
- bitget.go: Set TextPing=true, handle text 'pong', add debug logging
- scanner.go: Expand to 179 overlapping Bitget+HL coins

exchange/connector.go

@@ -16,6 +16,10 @@ type PriceConnector struct {
 	ReconnectBase time.Duration
 	PingInterval  time.Duration // 0 = no client-side pings
 
+	// TextPing: if true, sends text message "ping" instead of WebSocket PingMessage frame.
+	// Used by Bitget v2 WS which expects a JSON/text "ping" string.
+	TextPing bool
+
 	OnConnect func()
 	OnMessage func([]byte)
 	OnError   func(error)
@@ -91,8 +95,15 @@ func (pc *PriceConnector) Run() error {
 				for {
 					select {
 					case <-ticker.C:
-						if err := c.WriteMessage(websocket.PingMessage, nil); err != nil {
-							return
+						if pc.TextPing {
+							// Bitget v2 expects a TEXT message "ping", not WebSocket PingMessage frame
+							if err := c.WriteMessage(websocket.TextMessage, []byte("ping")); err != nil {
+								return
+							}
+						} else {
+							if err := c.WriteMessage(websocket.PingMessage, nil); err != nil {
+								return
+							}
 						}
 					case <-pingStop:
 						return