Remove the unused Flarum forum integration #9005

Task Summary

Texera can show a discussion forum backed by a separately installed Flarum instance (#2201). The feature is off by default (gui.tabs.forum_enabled = false), none of the deployment configs (single-node, k8s) set up Flarum or route /forum to it, the backend sync service was already removed in #3165, and the only changes since 2024 are a login-loop fix (#5311) and tests.

It also ships bin/forum/flarum.sql, a MySQL dump of a development Flarum database, under an ASF license header, although its schema and seed data come from Flarum. That came up while auditing third-party content in the source release for v1.3, so the removal should also go to release/v1.3.

Proposal: remove the integration.

• Frontend: FlarumComponent, FlarumService and their specs, the forum route and dashboard tab, the forum login on the dashboard, the flarum_remember cookie cleanup on logout, and the forum toggle in admin settings.
• Config: forum_enabled in default.conf, gui-config.ts, and the config-service specs.
• bin/: bin/forum/, bin/config.php, bin/.htaccess, and their rows in bin/README.md.
• Comments in JwtAuth.scala and OrcidAuthResource.scala that refer to Flarum.

If any deployment still uses the forum, please comment here.

Task Type

• Refactor / Cleanup
• DevOps / Deployment / CI
• Testing / QA
• Documentation
• Performance
• Other