fix: handle SIGTERM gracefully for clean container shutdown

Register signal handler on the event loop to stop the server
on SIGTERM/SIGINT instead of relying on serve_forever().

src/s5p/server.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 
 import asyncio
 import logging
+import signal
 import struct
 import time
 
@@ -231,5 +232,12 @@ async def serve(config: Config) -> None:
     else:
         logger.info("  mode: direct (no chain)")
 
+    loop = asyncio.get_running_loop()
+    stop = loop.create_future()
+
+    for sig in (signal.SIGTERM, signal.SIGINT):
+        loop.add_signal_handler(sig, lambda s=sig: stop.set_result(s))
+
     async with srv:
-        await srv.serve_forever()
+        sig = await stop
+        logger.info("received %s, shutting down", signal.Signals(sig).name)