🐛 fix: improve graceful shutdown handling

[sliver2er]

Changes

• Track SSE clients: All SSE connections are now tracked in a Set
• Broadcast shutdown event: Before closing the server, all SSE clients receive a 'shutdown' event
• Destroy active sockets: Active HTTP connections are destroyed on shutdown to prevent server hanging
• Prevent duplicate shutdown: Added isShuttingDown flag to prevent multiple shutdown calls from Ctrl+C presses
• Immediate termination: Server now closes immediately on first Ctrl+C instead of requiring multiple presses

Problem

Previously, the server would hang on shutdown because:

1. SSE (Server-Sent Events) connections remained open indefinitely
2. server.close() would wait for all connections to close before triggering the callback
3. This callback never fired, so the process never exited
4. Users had to press Ctrl+C multiple times to force shutdown

Solution

1. Track all SSE client connections in a Set
2. When SIGINT/SIGTERM is received, broadcast a 'shutdown' event to all SSE clients
3. Immediately close all SSE connections
4. Destroy any remaining HTTP sockets
5. Call process.exit() after closing the server
6. Use isShuttingDown flag to prevent duplicate shutdown handlers

Testing

npm run build
npx viewcc
# Press Ctrl+C once - server should shut down immediately
# No longer requires multiple Ctrl+C presses