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.

The official image has no *_FILE support, so the wrapper entrypoint reads both
the db_password and smtp_password secrets and exports DISCOURSE_DB_PASSWORD /
DISCOURSE_SMTP_PASSWORD before booting. SMTP env vars are renamed to the
official names; release notes document the migration for operators.

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).

compose.smtpauth.yml

@@ -3,14 +3,8 @@ version: "3.8"
 
 services:
   app:
-    environment:
-      - DISCOURSE_SMTP_PASSWORD_FILE=/var/run/secrets/smtp_password
-    secrets:
-      - smtp_password
-
-  sidekiq:
-    environment:
-      - DISCOURSE_SMTP_PASSWORD_FILE=/var/run/secrets/smtp_password
+    # the wrapper entrypoint reads /run/secrets/smtp_password and exports
+    # DISCOURSE_SMTP_PASSWORD (the official image has no *_FILE support)
     secrets:
       - smtp_password