docs: Add docs for the websocket ping interval configuration (#407)

Author: marcelomendoncasoares

docs/06-concepts/07-configuration.md

@@ -16,7 +16,7 @@ flowchart TB
     A --> B
     B --> C
 
-    %% Styles (optional – tweak to your theme)
+    %% Styles (optional - tweak to your theme)
   class A yaml
     class B env
     class C dart
@@ -69,6 +69,7 @@ These can be separately declared for each run mode in the corresponding yaml fil
 | SERVERPOD_FUTURE_CALL_EXECUTION_ENABLED  | futureCallExecutionEnabled    | true      | Enables or disables the execution of future calls.                                                                                                    |
 | SERVERPOD_FUTURE_CALL_CONCURRENCY_LIMIT  | futureCall.concurrencyLimit   | 1         | The maximum number of concurrent future calls allowed. If the value is negative or null, no limit is applied.                                         |
 | SERVERPOD_FUTURE_CALL_SCAN_INTERVAL      | futureCall.scanInterval       | 5000      | The interval in milliseconds for scanning future calls                                                                                                |
+| SERVERPOD_WEBSOCKET_PING_INTERVAL        | websocketPingInterval         | 30        | The interval in seconds between WebSocket ping messages sent to keep streaming connections alive. Must be a positive integer.                         |
 
 ### Secrets
 
@@ -165,6 +166,7 @@ apiServer:
   publicHost: localhost
   publicPort: 8080
   publicScheme: http
+  websocketPingInterval: 30
 
 insightsServer:
   port: 8081

docs/06-concepts/15-streams.md

@@ -79,6 +79,12 @@ Authentication is validated when the stream is first established, utilizing the
 
 For more details on handling revoked authentication, refer to the section on [handling revoked authentication](authentication/custom-overrides#handling-revoked-authentication).
 
+### WebSocket ping interval
+
+The server sends periodic ping messages on open streaming connections to keep them alive. The interval between pings is configurable and defaults to 30 seconds.
+
+If you deploy behind a load balancer or proxy with a shorter idle timeout (for example, 15-20 seconds), you may need to lower the ping interval so connections are not closed. Set the `SERVERPOD_WEBSOCKET_PING_INTERVAL` environment variable to the desired interval in seconds, or configure `websocketPingInterval` in your [configuration](./configuration) file.
+
 ### Error handling
 
 Error handling works just like in regular endpoint methods in Serverpod. If an exception is thrown on a stream, the stream is closed with an exception. If the exception thrown is a serializable exception, the exception is first serialized and passed over the stream before it is closed.
@@ -110,8 +116,8 @@ outStream.listen((message) {
 inStream.addError(SerializableException('Error from client'));
 
 // This will print
-// Server received error: Error from client 
-// Client received error: Error from server 
+// Server received error: Error from client
+// Client received error: Error from server
 ```
 
 In the example above, the client sends an error to the server, which then throws an exception back to the client. And since the exception is serializable, it is passed over the stream before the stream is closed.