✨(collaboration) notify the backend when the worker persists new content

notify the backend when the worker persists new content for
a document, so the lists ordered by `updated_at` follow the edits made on the
collaboration server. The backend serves it on
`POST /api/v1.0/documents/{id}/content-updated/`, authenticated with a short
lived RS256 JWT the collaboration server signs (`aud: "docs-backend"`) and
the backend verifies against the JWKS the collaboration server publishes on
`/collaboration/jwks/v1` — the mirror of the admin token the backend signs to
call it, so no long lived secret is shared and either side can roll its key
on its own

compose.yml

@@ -246,9 +246,13 @@ services:
       # seed rooms from the legacy Django/S3 document store on first access —
       # S3 endpoint/credentials come from env.d/development/common
       SOFT_MIGRATION: "true"
+      # signs the calls made to the backend, which holds the public half
+      YHUB_JWT_PRIVATE_KEY_FILE: /data/jwt/yhub-private.pem
     env_file:
       - env.d/development/common
       - env.d/development/common.local
+    volumes:
+      - ./data/jwt:/data/jwt:ro
     restart: unless-stopped
     ports:
       - "3002:3002"