Fix misleading keepalive comment in PHP server

The comment said keepalives were sent every 15 seconds, but the code
uses sleep(1), so they're actually sent every 1 second. This is correct
for SSE connections - frequent keepalives prevent proxy timeouts.

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Co-Authored-By: Claude <noreply@anthropic.com>

server/php/server.php

@@ -128,7 +128,7 @@ function handleStream() {
             $lastCount = $currentCount;
         }
 
-        // Send keepalive comment every 15 seconds
+        // Send keepalive comment every 1 second (keeps SSE connection alive)
         echo ": keepalive\n\n";
         flush();