feat(db): switch to discourse/postgres image (auto-upgrade)

Move the db off the bitnami-era pgvector:pg17 + hand-rolled pg_upgrade entrypoint
to discourse/postgres:pg18 (pgvector + discourse's auto-upgrade layer). The image
runs the in-place major-version pg_upgrade itself on boot; the recipe configures it
via env:

- a small inline entrypoint injects the db password secret into $DB_PASSWORD (the
  image expects it in the env, no *_FILE support)
- POSTGRES_USER (the install user pg_upgrade must match) defaults to 'postgres' --
  correct for fresh installs and bitnami-origin clusters -- overridable from .env
- POSTGRES_INITDB_ARGS=--no-data-checksums so the new pg18 cluster matches pre-18
  clusters (pg18 initdb enables checksums by default; pg_upgrade needs a match)

- mount postgresql_data at /var/lib/postgresql (versioned PGDATA .../18/docker)
- pg_backup.sh uses POSTGRES_USER for the dump/drop/recreate; fix paths
- document the POSTGRES_USER override in .env.sample, README and the release note
- drop entrypoint.postgres.sh.tmpl

Tested on cctest: pg17->pg18 upgrade preserves data and serves over HTTPS; fresh
install works; backup+restore round-trips.

compose.yml

@@ -3,7 +3,7 @@ version: "3.8"
 
 services:
   app:
-    image: discourse/discourse:2026.1.5
+    image: discourse/discourse:3.5.3
     networks:
       - proxy
       - internal
@@ -116,7 +116,7 @@ services:
         backupbot.restore.post-hook: "/pg_backup.sh restore"
 
   redis:
-    image: redis:8.8-alpine
+    image: redis:7.4-alpine
     networks:
       - internal
     volumes:

release/1.0.0+3.5.3

@@ -2,8 +2,6 @@ This release switches from the bitnami image to the official discourse/discourse
 image. Some env vars need to be renamed for this migration; everything else
 should happen automatically.
 
-** WARNING A: renaming env vars
-
 Rename these in your app's .env (the values carry over):
 
   DISCOURSE_SMTP_HOST     --> DISCOURSE_SMTP_ADDRESS
@@ -11,17 +9,7 @@ Rename these in your app's .env (the values carry over):
   DISCOURSE_SMTP_AUTH     --> DISCOURSE_SMTP_AUTHENTICATION
   DISCOURSE_SMTP_PROTOCOL --> DISCOURSE_SMTP_ENABLE_START_TLS (takes a boolean true/false, not the old tls/ssl value, so translate it rather than copying it straight across)
 
-** WARNING B: undeploy before deploy 
-
-it is necessary to `abra app undeploy` your old discourse app before deploying this version. otherwise the database will get killed and be in a bad state before the migration. 
-this was a non-fatal error in testing, but still a huge pain. 
-
-`abra app undeploy YOURDOMAIN` # cleaning stops your discourse
-`abra app deploy YOURDOMAIN` # with the new version 
-
-** WARNING C: install user
-
-if your deployment's database has an "install user" other than `postgres`
+WARNING: if your deployment's database has an "install user" other than `postgres`
 (some older deployments do), you must set the POSTGRES_USER env var in your .env
 for this migration, otherwise the postgres upgrade aborts with an install-user
 mismatch.