feat(talk/turn/signaling/hpb): add role for Talk with backend services

inventories/demo-gymburgdorf/host_vars/turn/traefik.yml

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+# Local Traefik on the turn host (docker provider mode) picks up
+# container labels from the signaling stack. coturn lives next to it
+# via host networking on 3478/5349 — no port collision.
+traefik_mode: "backend"
+traefik_network: "proxy"
+
+# Tell the DMZ reverseproxy to expose this host for signaling.digitalboard.ch
+# (DMZ proxy aggregates traefik_dmz_exposed_services from all backend_servers).
+traefik_dmz_exposed_services:
+  - name: signaling
+    domain: signaling.digitalboard.ch
+    port: 443
+    protocol: https