feat: add community posts backend

[Arcadi4]

Summary

• Add a standalone posts backend app for community posts.
• Support authenticated post creation, reading, editing, and soft deletion with author-owned write permissions.
• Integrate community posts into the shared content-target system so they can receive comments, reactions, and content reports.
• Add markdown mention support for community posts with rendered mention output and current username resolution.

Backend

• Add the CommunityPost model, admin, serializers, services, permissions, routes, migrations, and tests.
• Register the posts app in backend settings, API routing, backend docs, and CI lint scope.
• Expose post read fields for content interactions, including:
  • content_target
  • render_body
  • mentions
  • mention_users
  • comment_count
  • like_count
  • dislike_count
  • my_reaction

Content Targets

• Extend ContentTarget with a COMMUNITY_POST target type and a strong one-to-one link to CommunityPost.
• Eagerly create content targets for published articles, comments, and community posts through shared core signals.
• Keep content-target helpers idempotent for service and API workflows.

Comments And Reactions

• Allow comments to be created and filtered directly by community_post.
• Return the related community post id from comment responses when applicable.
• Allow reactions to be created and cleared directly for community posts.
• Add community post reaction and comment summary query support.

Mentions And Reports

• Reuse the shared markdown mention validation and rendering utilities for community post bodies.
• Store ordered mention user ids on community posts and return resolved mention users in read responses.
• Extend content report snapshots to preserve community post bodies, rendered mentions, mention ids, author ids, and deletion state.

Tests

• Cover community post models, serializers, services, permissions, and API behavior.
• Cover community post mentions, content-target creation, comment integration, reaction integration, and report snapshots.
• Cover eager content-target creation when targetable models are created directly through the ORM.

Closes #64