feat(discourse): switch app to official discourse/discourse image (experimental)

Replaces the paywalled bitnamilegacy app with the official discourse/discourse
image behind Traefik (HTTP-only via an install-ssl override; sidekiq is internal
so its service is dropped). DB is reused as-is; uploads migrate from a legacy
bitnami volume via an idempotent, non-destructive runit hook. db keeps
pgvector/pgvector:pg17 with the install-user-aware pg_upgrade entrypoint, plus
db/redis healthchecks and updated README metadata (lint R002/R007).

Verified on cctest: fresh install, upgrade-from-bitnami-pg17, and
upgrade-from-bitnami-pg13 (incl. 13->17) all serve with data intact.
Upstream marks discourse/discourse experimental; hold prod cutover.

Recipe 0.8.1+3.5.0 -> 1.0.0+3.5.3 (major: new image, env/volume/port changes).

.env.sample

@@ -5,17 +5,19 @@ DOMAIN=discourse.example.com
 #EXTRA_DOMAINS=', `www.discourse.example.com`'
 LETS_ENCRYPT_ENV=production
 
-# Outgoing email
-#DISCOURSE_SMTP_HOST=
-#DISCOURSE_SMTP_PORT=
-#DISCOURSE_SMTP_USER=
-#DISCOURSE_SMTP_PROTOCOL=
-#DISCOURSE_SMTP_AUTH=
-# Set this if you send e-mails from a different domain than noreply@$DOMAIN
-#DISCOURSE_NOTIFICATION_EMAIL=$SMTP_USER
+# Admin / developer accounts (comma-separated); these become admins on signup
+DISCOURSE_DEVELOPER_EMAILS=admin@example.com
 
-# SMTP authentication
-#COMPOSE_FILE="compose.yml:compose.smtpauth.yml"
+# Outgoing email (official discourse/discourse env names)
+#DISCOURSE_SMTP_ADDRESS=
+#DISCOURSE_SMTP_PORT=587
+#DISCOURSE_SMTP_USER_NAME=
+#DISCOURSE_SMTP_AUTHENTICATION=login
+#DISCOURSE_SMTP_ENABLE_START_TLS=true
+# Set this if you send e-mail from a different address than noreply@$DOMAIN
+#DISCOURSE_NOTIFICATION_EMAIL=
+
+# SMTP password as a secret
 #SECRET_SMTP_PASSWORD_VERSION=v1
 
 SECRET_DB_PASSWORD_VERSION=v1