upstream(mattermost-lts): release-notes sources

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cc-ci-plan/upstream/mattermost-lts.md

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+# Upstream sources — mattermost-lts
+
+| service  | image                                     | source repo                                       | releases / changelog                                              |
+|----------|-------------------------------------------|---------------------------------------------------|-------------------------------------------------------------------|
+| app      | mattermost/mattermost-team-edition        | https://github.com/mattermost/mattermost          | https://docs.mattermost.com/about/mattermost-changelog.html       |
+| postgres | postgres                                  | https://github.com/postgres/postgres              | https://www.postgresql.org/docs/release/                          |
+
+## Standing notes
+- mattermost-lts tracks the **10.x LTS series** — do NOT bump to 11.x (major version, breaking changes).
+  The latest 10.x tag is on `mattermost/mattermost-team-edition` e.g. `10.11.x`; check release notes
+  at https://docs.mattermost.com/about/mattermost-changelog.html.
+- postgres: stay on **15-alpine** for now. The upgrade to 16-alpine requires a major-version migration
+  (pg_upgrade or dump/restore with operator involvement) because the stack runs with a persistent
+  postgres_data volume. A 16 container against pg15 PGDATA crashes on startup. Major pg bump is a
+  separate operator-guided step.
+- The backup/restore hooks are INLINED in compose.yml `deploy.labels` (not a separate pg_backup.sh
+  Docker config). This was intentional: using a Docker config caused a timing race where backupbot
+  could exec into the OLD postgres container (pre-config-update) during the upgrade lifecycle. Inline
+  commands eliminate this dependency — the backupbot reads labels from the Docker service API
+  (always current), and pg_dump/psql/createdb are present in any postgres:alpine container.