feat(backend): add graceful shutdown for HTTP and WebSocket servers (#170)

[mattcrypted]

Summary

Implements #170: graceful shutdown for the HTTP and WebSocket servers. On
SIGTERM or SIGINT the process stops accepting new traffic, drains in-flight
requests within a configurable grace period, closes the WebSocket and Redis
connections, and exits with code 0 on a clean drain or 1 on timeout.

Changes

• New src/utils/shutdown.ts: a dependency-injected shutdown coordinator.
  • Runs ordered cleanup steps; a step that throws is logged and the rest still run.
  • Grace period via GRACEFUL_SHUTDOWN_TIMEOUT_MS (default 30000); the sequence is raced against this deadline so the process always exits within the grace window.
  • shutdownGuard middleware returns 503 for new requests during drain, exempting /api/health and / so orchestrators can still read state.
  • isShuttingDown() flag and closeHttpServer() helper (drains in-flight, frees idle keep-alive sockets).
• src/index.ts: replaced the partial SIGINT-only stub with registerShutdownHandlers covering SIGTERM and SIGINT; registered steps for WebSocket close, HTTP drain, transaction queue/processor/events, and Redis; /api/health now returns 503 { status: 'shutting down' } during drain.
• src/services/websocketService.ts: added close() which emits a server:shutdown notice to clients and force-disconnects sockets so the shared HTTP server can finish draining.
• src/tests/shutdown.test.ts: 10 unit tests.

Definition of Done

• SIGTERM and SIGINT handlers registered
• HTTP server stops accepting new connections; new requests get 503
• In-flight requests drain within a configurable grace period (default 30s)
• Socket.IO closed with a "server shutting down" message
• Redis closed gracefully (no Mongoose in this codebase; pg pools are created ad-hoc with no shared lifetime pool to close)
• Health check reports "shutting down" during drain
• Exit code 0 on clean shutdown, 1 on timeout

Testing

• 10/10 unit tests pass; 100% line coverage of src/utils/shutdown.ts (98% statements).
• Tests run with a setup-free jest config because main currently does not compile (pre-existing TypeScript errors in src/index.ts and other files, unrelated to this change; tests/setup.js imports src/index.ts). This change adds zero new tsc or eslint problems.

Closes #170