build(cms): wire Payload into the Docker image and both stacks

Uploads go to a named volume at /app/media. The directory is created in
the image before the mount and covered by the existing chown, because
Docker seeds a fresh named volume from the image path — a missing or
root-owned directory there fails every upload with EACCES at runtime,
long after the build passed.

PAYLOAD_SECRET uses the same :? form as AIRFLOW_ADMIN_PASSWORD: refuse
to start rather than boot with an empty secret and accept forged
sessions. Staging's must differ from production's, which the header
comment now says explicitly. Portainer prefixes volume names per stack,
so payload_media isolates itself.

prodMigrations is not wired yet — generating the initial migration needs
a reachable Postgres. Follows in its own commit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017YmbBhr8s7GusjDE12hrZM

docker-compose.portainer.yml

@@ -9,6 +9,9 @@
 #   TYPESENSE_SEARCH_KEY   — Typesense search-only key (exposed to frontend)
 #   UNLEASH_URL            — http://<unleash-ip>:4242/api (empty = all flags off)
 #   UNLEASH_API_TOKEN      — Unleash *client* token, environment: production
+#   PAYLOAD_SECRET         — Payload CMS encryption secret. REQUIRED: long and
+#                            random. Changing it invalidates every admin
+#                            session. Staging MUST use a different value.
 #   AIRFLOW_ADMIN_USER     — Airflow admin username (default: admin)
 #   AIRFLOW_ADMIN_PASSWORD — Airflow admin password. REQUIRED: the api-server
 #                            refuses to start without it, rather than falling
@@ -78,9 +81,21 @@ services:
       - FASTAPI_URL=http://backend:80/api
       - TYPESENSE_URL=http://typesense:8108
       - TYPESENSE_API_KEY=${TYPESENSE_SEARCH_KEY:-changeme}
+      # Payload CMS runs inside this container. It reaches Postgres over the
+      # `backend` network and keeps its tables in the `payload` schema, so no
+      # pipeline operation on `public` can touch blog content.
+      - DATABASE_URL=postgresql://${DB_USERNAME}:${DB_PASSWORD}@sc_database:5432/${DB_DATABASE_NAME}
+      # Same :? form as AIRFLOW_ADMIN_PASSWORD: refuse to start rather than
+      # boot with an empty secret and silently accept forged sessions.
+      - PAYLOAD_SECRET=${PAYLOAD_SECRET:?set PAYLOAD_SECRET in the Portainer stack environment}
+    volumes:
+      # Blog images. Not reproducible from the pipeline — must be backed up.
+      - payload_media:/app/media
     depends_on:
       backend:
         condition: service_healthy
+      sc_database:
+        condition: service_healthy
     networks:
       backend: {}
       macvlan:
@@ -231,3 +246,4 @@ volumes:
   typesense_data:
   airflow_logs:
   unleash_cache:
+  payload_media: