Revert to headed Chrome + Xvfb; add shm_size 256m to fix Xvfb in Docker

Headless mode gets blocked by Cloudflare on the search endpoint. Headed
mode with Xvfb works on dev (Unraid). The missing piece on Proxmox LXC was
insufficient /dev/shm (Docker default 64 MB); setting shm_size: 256m gives
Xvfb enough shared memory to start.

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

docker-compose.yml

@@ -3,6 +3,7 @@ services:
     build: ./backend
     security_opt:
       - apparmor=unconfined
+    shm_size: '256m'
     environment:
       - DATABASE_URL=${DATABASE_URL}
       - CENTRAL_AUTH_SECRET=${CENTRAL_AUTH_SECRET}