fix(server): respect externally set PAPERCLIP_API_URL env var

Previously the server unconditionally overwrote PAPERCLIP_API_URL on
startup, clobbering any value set externally (e.g., via Kubernetes
ConfigMap). Now only sets the default if the env var is not already set.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/index.ts

@@ -553,7 +553,9 @@ export async function startServer(): Promise<StartedServer> {
       : runtimeListenHost;
   process.env.PAPERCLIP_LISTEN_HOST = runtimeListenHost;
   process.env.PAPERCLIP_LISTEN_PORT = String(listenPort);
-  process.env.PAPERCLIP_API_URL = `http://${runtimeApiHost}:${listenPort}`;
+  if (!process.env.PAPERCLIP_API_URL) {
+    process.env.PAPERCLIP_API_URL = `http://${runtimeApiHost}:${listenPort}`;
+  }
   
   setupLiveEventsWebSocketServer(server, db as any, {
     deploymentMode: config.deploymentMode,