feat(webchat): embeddable Web Chat widget as a channel connection

Web Chat is a WEB connection of a processing channel (SPEC-0003/ADR-0012),
reusing the whole contour: a public widget loads from the hub domain via one
async tag (<script src=hub.edevs.tech/chat-widget.js data-channel=...>), opens
an isolated iframe panel. Backend webchat app: public config/session/messages
endpoints (anonymous session, hash-at-rest token); inbound goes through the same
ingest (AI release + retrieval + handoff + notifications); WEB transport send is
a no-op (browser polls). Panel rebuilt 1:1 from the baseline (welcome+consent /
ai / operator / unavailable, quick replies, typing). nginx serves /chat-widget.js
and /chat/ from the hub domain; demo host page at /chat/demo.html. Verified
end-to-end: config→session→message→AI reply→poll.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

deploy/nginx/local.conf

@@ -9,6 +9,20 @@ server {
     proxy_set_header X-Forwarded-Proto $scheme;
   }
 
+  # Web Chat widget loader (served from the hub domain).
+  location = /chat-widget.js {
+    proxy_pass http://backend:8000;
+    proxy_set_header Host $host;
+  }
+
+  # Web Chat panel (iframe content) — Vite base /chat/.
+  location /chat/ {
+    proxy_pass http://web-chat:5175;
+    proxy_set_header Host $host;
+    proxy_set_header Upgrade $http_upgrade;
+    proxy_set_header Connection "upgrade";
+  }
+
   location / {
     proxy_pass http://internal-ui:5173;
     proxy_set_header Host $host;