fix: add shutdown timeout so cProfile data is written on SIGTERM

srv.wait_closed() blocked indefinitely on active relay connections,
preventing serve() from returning and prof.dump_stats() from running.

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

src/s5p/server.py

@@ -528,7 +528,12 @@ async def serve(config: Config) -> None:
         for srv in servers:
             srv.close()
         for srv in servers:
-            await srv.wait_closed()
+            try:
+                await asyncio.wait_for(srv.wait_closed(), timeout=5.0)
+            except TimeoutError:
+                pass
+        if metrics.active:
+            logger.info("shutdown: %d connections still active", metrics.active)
         if api_srv:
             api_srv.close()
             await api_srv.wait_closed()