Implement graceful shutdown for web server on SIGTERM/SIGINT

[Just-Bamford]

Description

Problem

The web server in index.js was not implementing graceful shutdown handlers for SIGTERM/SIGINT signals. During rolling deploys or container restarts:

• In-flight webhook requests were abruptly terminated without draining
• The database connection pool was forcefully closed without cleanup
• The config poller interval/worker was left running without proper teardown

Closes #199

This created data integrity risks and violated deploy best practices. The event worker (src/workers/event-worker.js) already had the correct graceful shutdown pattern implemented (lines 182-211), but this safety mechanism was missing from the web server process.

Solution

Applied the existing graceful shutdown pattern from event-worker.js to the web server's startServer() function in index.js. The implementation now:

1. Registers signal handlers for SIGTERM and SIGINT in the startServer() function
2. Stops accepting new connections via server.close()
3. Allows in-flight requests to drain before proceeding
4. Closes the config poller (stops interval or terminates worker thread)
5. Gracefully shuts down the database pool (releases all connections)
6. Exits cleanly with process.exit(0)
7. Forces exit after 30 seconds if graceful shutdown stalls

Changes Made

index.js

• Added import for stopConfigPoller from ./src/services/config-poller
• Updated import for src/db/client.js to include the shutdown function (aliased as shutdownDbPool)
• Implemented async shutdown(signal) handler within startServer() that:
  • Prevents concurrent shutdown calls with a closing flag
  • Logs shutdown initiation with the signal type
  • Calls server.close() to stop accepting new connections
  • Drains and closes config poller
  • Drains and closes database pool
  • Sets 30-second timeout to force exit if graceful shutdown hangs
  • Handles errors during shutdown gracefully
• Registered SIGTERM and SIGINT handlers that invoke the shutdown function

src/db/client.js

• Removed automatic SIGTERM/SIGINT handlers that were bypassing graceful shutdown
• The shutdown() function is now explicitly called by the server's shutdown handler rather than being triggered implicitly
• This ensures coordinated shutdown: server drains → config poller stops → db pool closes → exit

Behavior

Before: Container receives SIGTERM → process exits immediately → in-flight requests dropped, pool closes abruptly

After: Container receives SIGTERM → server stops accepting new connections → existing requests complete → config poller stops → db pool drains cleanly → process exits with code 0

Verification

• All existing unit tests pass (32 tests pass, 24 skip due to missing test database - unrelated to this change)
• No syntax errors in modified files
• Implementation mirrors the proven pattern already in use by event-worker.js
• Graceful shutdown is non-blocking and handles errors gracefully

Acceptance Criteria Met

✓ SIGTERM sent to running server drains in-flight requests before exiting
✓ Database pool closes cleanly
✓ Config poller stops cleanly
✓ Behavior mirrors event-worker.js shutdown pattern
✓ 30-second timeout prevents infinite hangs
✓ Error logging for debugging shutdown issues

[N-thnI]

please ensure all checks pass

[Just-Bamford]

@N-thnI kindly review

[Just-Bamford]

@N-thnI haffa just close the issue