Fix timeout middleware panic on proxy/WebSocket routes and worker healthcheck

The TimeoutMiddleware wraps the response writer in *http.timeoutWriter which
doesn't implement http.Flusher. When the admin reverse proxy or WebSocket
upgrader tries to flush, it panics and crashes the container (502 Bad Gateway).
Skip timeout for /admin, /_next, and /ws routes.

Also fix the Dockerfile HEALTHCHECK to detect the worker process — the worker
has no HTTP server so the curl-based check always failed, marking it unhealthy.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -145,6 +145,6 @@ RUN mkdir -p /app/uploads
 EXPOSE 5000
 
 HEALTHCHECK --interval=30s --timeout=10s --start-period=10s --retries=3 \
-    CMD curl -f http://localhost:${PORT:-5000}/api/health/ || exit 1
+    CMD pgrep -x worker > /dev/null && exit 0 || curl -f http://localhost:${PORT:-5000}/api/health/ || exit 1
 
 CMD ["/app/start.sh"]