agent window: a site's relay stands in for the endpoint it relays

On isokit.buildfor.life, entering https://inference.p0.contact/v1 in the
agent window's settings made the page call the inference server directly,
which browsers refuse (its nginx answers the CORS preflight with 401):
"The browser could not reach ... (Failed to fetch)".

- the relay meta names its upstream (data-upstream; build_web.py
  --agent-upstream, the deploy passes the inference server): that URL now
  goes through the relay and is saved as "follow the site's default"
- an unreachable endpoint says what to do: the relay's URL on a site with
  one, else isokit studio or isokit.buildfor.life
- check_web.py: relayed upstream and the unreachable message

.gitea/workflows/deploy-web.yml

@@ -80,7 +80,7 @@ jobs:
         run: |
           # the agent window's default endpoint: the site's relay to B4L's
           # inference server (deploy/Caddyfile, handle_path /llm/*)
-          python3 tools/build_web.py --agent-url /llm/v1
+          python3 tools/build_web.py --agent-url /llm/v1 --agent-upstream https://inference.p0.contact/v1
           echo "$GITHUB_SHA" > dist/web/version.txt
 
       - name: Set up SSH