refactor(backend): extract remote proxy, WebSocket upgrade handler, and server factory (phase 3) (#733)

Phase 3 of the index.ts refactor. Pulls the remote HTTP/WS proxy plumbing,
the WebSocket upgrade dispatcher, and the http/WSS construction out of the
monolith. index.ts drops roughly 620 lines.

New modules:
- proxy/websocketProxy.ts: shared httpProxy.createProxyServer singleton
  (used by both the HTTP proxy middleware and the remote WS forwarder)
- proxy/remoteNodeProxy.ts: createRemoteProxyMiddleware() factory; consumes
  the isProxyExemptPath helper instead of open-coding the prefix list
- server.ts: createServer(app) returns { server, wss, pilotTunnelWss }
- services/FleetUpdateTrackerService.ts: singleton wrapping the in-flight
  fleet update tracker Map with create()/resolve() helpers
- helpers/consoleSession.ts: mintConsoleSession(), isConsoleSessionScope()
- websocket/upgradeHandler.ts: attachUpgrade(server, deps) dispatcher that
  runs the manual cookie/JWT verify and delegates to sub-handlers
- websocket/pilotTunnel.ts: handlePilotTunnel (pilot_enroll consumption and
  pilot_tunnel registration)
- websocket/notifications.ts: /ws/notifications local subscriber
- websocket/remoteForwarder.ts: remote-node WS proxy with console_session
  token exchange for interactive paths
- websocket/logs.ts: /api/stacks/:name/logs supervisor stream
- websocket/hostConsole.ts: /api/system/host-console PTY, Admiral-gated
- websocket/generic.ts: /ws exec + streamStats action dispatch, owns the
  terminalWs single-instance reference
- websocket/reject.ts: shared rejectUpgrade helper (replaces five copies)

Service extension:
- NotificationService: setBroadcaster(fn) replaced by subscribe(ws) that
  returns an unsubscriber; broadcastToSubscribers is now internal. Subscriber
  set lives on the service rather than in index.ts.

Wiring in index.ts:
- const app = createApp() already in place from Phase 2
- const { server, wss, pilotTunnelWss } = createServer(app)
- attachUpgrade(server, { wss, pilotTunnelWss })
- app.use('/api/', createRemoteProxyMiddleware())
- /api/system/console-token route now uses mintConsoleSession()
- deploy/down/update routes read the streaming target via getTerminalWs()
  (return type is WebSocket | undefined so the || undefined fallback is gone)

Code review fixes: five duplicated reject helpers collapsed into
websocket/reject.ts; dropped the createTracker/resolveTracker bind
aliases in index.ts so call sites go through the service directly;
removed em dashes; replaced req.url! with req.url || '/'.

backend/src/app.ts

@@ -26,7 +26,7 @@ import './types/express';
  *   10. authGate (at /api)                -- registered in index.ts (before Phase 4 finishes)
  *   11. auditLog (at /api)                -- registered in index.ts
  *   12. enforceApiTokenScope (at /api)    -- registered in index.ts
- *   13. remote HTTP proxy                 -- Phase 3
+ *   13. createRemoteProxyMiddleware       -- proxy/remoteNodeProxy.ts, registered in index.ts
  *   14. routes                            -- registered in index.ts (moves to routes/* in Phase 4)
  *   15. static serving + SPA fallback     -- registered in index.ts (moves here in Phase 5)
  *   16. errorHandler                      -- registered in index.ts