ci(deploy): probe backend /readyz so a dead backend fails the deploy

The post-deploy health check probed only the static landing (`GET /`) and the
gateway-served SPA shell (`GET /app/`) through the edge caddy. Neither needs the
backend, so a crash-looping backend passed the deploy green — which masked the
seed-drift crash until it was inspected by hand.

Add a third probe, `http://backend:8080/readyz` on the internal network, to the
same retry loop, and dump the backend logs on failure. A backend that cannot
become ready now fails the deploy loudly instead of going green.

.gitea/workflows/ci.yaml

@@ -299,22 +299,28 @@ jobs:
           # pick up the fresh config.
           docker compose --ansi never up -d --force-recreate --no-deps caddy otelcol prometheus tempo grafana
 
-      - name: Probe the landing and the gateway through caddy
+      - name: Probe the landing, gateway and backend
         run: |
           set -u
-          # Two probes through the contour caddy: "/" is the static
-          # landing container, "/app/" is the gateway-served SPA shell.
+          # Three probes. "/" is the static landing container and "/app/" the
+          # gateway-served SPA shell (both through the contour caddy on the edge net).
+          # The backend /readyz is probed on the internal net as well: the caddy probes
+          # are blind to a crash-looping backend (the landing is static and the SPA
+          # shell is served without it), which let a bad deploy go green while the
+          # backend was down — so check it directly here.
           for i in $(seq 1 20); do
             if docker run --rm --network edge alpine:3.20 wget -q -T 5 -O /dev/null http://scrabble/ &&
-               docker run --rm --network edge alpine:3.20 wget -q -T 5 -O /dev/null http://scrabble/app/; then
-              echo "healthy: GET http://scrabble/ (landing) + /app/ (gateway)"
+               docker run --rm --network edge alpine:3.20 wget -q -T 5 -O /dev/null http://scrabble/app/ &&
+               docker run --rm --network scrabble-internal alpine:3.20 wget -q -T 5 -O /dev/null http://backend:8080/readyz; then
+              echo "healthy: GET / (landing) + /app/ (gateway) + backend /readyz"
               exit 0
             fi
             sleep 3
           done
-          echo "probe failed; recent landing + gateway logs:"
+          echo "probe failed; recent landing + gateway + backend logs:"
           docker logs --tail 50 scrabble-landing || true
           docker logs --tail 50 scrabble-gateway || true
+          docker logs --tail 50 scrabble-backend || true
           exit 1
 
       - name: Probe the Telegram connector liveness