Add post pinning feature for project feeds

[SylvainChevalier]

Summary

This PR adds the ability for project admins to pin posts to the top of their project's feed, similar to the existing comment pinning feature.

Key Changes

Backend:

• Added is_pinned boolean field to the Post model with database indexing
• Implemented pin_post() and unpin_post() service functions in posts/services/common.py
• Created post_toggle_pin_api_view() endpoint that requires admin permissions to pin/unpin posts
• Updated get_posts_feed() to apply pinned post ordering logic:
  • Pinned posts appear at the top of their default project's feed
  • Pinning only applies when the feed is scoped to a single project (tournament, community, or explicit default project)
  • Pinning is ignored for unscoped feeds (main feed) and search/similarity results
  • Uses Django Case/When annotation to prioritize pinned posts while maintaining secondary sort order
• Added can_pin_post() permission check to ObjectPermission class
• Added URL route for the toggle-pin endpoint

Frontend:

• Added pin/unpin toggle to the post dropdown menu (admin-only)
• Displays a thumbtack icon on pinned posts in the post card
• Implemented optimistic UI updates with error handling
• Added togglePinPost() server action and API method
• Added i18n messages for pin/unpin actions and notifications
• Updated Post type to include is_pinned field

Tests:

• Added three comprehensive test cases covering:
  • Pinned posts appearing above unpinned posts in project feeds
  • Pinning only affecting the default project feed (not repost feeds)
  • Pinning being ignored in unscoped feeds
• Added integration test for the toggle-pin API endpoint with permission checks

Implementation Details

• Pinning respects the post's default_project relationship, ensuring pins only apply to the project where the post was originally created
• The feature uses a Case/When annotation to add a priority field (is_pinned_first) that sorts pinned posts first, then applies the requested ordering
• Admin permission is enforced at the API level using the existing permission system

https://claude.ai/code/session_01T4pYzdkyDaxzUry6iQP684

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[SylvainChevalier]

Heads up on the one red check — it's infra, not the code.

All code-relevant checks are green: Backend Checks, Frontend Checks, integration-tests, Email Template Checks, CodeQL (python/actions/js), and Docker build all pass.

The only failure is Deploy Preview App, and it's unrelated to this diff. Fly's abuse filter rejects the auto-generated preview app name:

Creating new Fly app: metaculus-pr-5119-claude-github-issue-3223-kip30
Error: ... Name blocked by abuse filter. GitHub is a common phishing target,
please select a new app name that doesn't contain GitHub

The app name is derived from the branch name (claude/github-issue-3223-kip300), which contains the substring github. Any branch with github in its name will hit this. I'm keeping the assigned branch name, so I'm not fixing this from the code side — it would need either a branch rename or a tweak to the preview workflow's app-name derivation (e.g. strip/replace github when building APP_NAME). Flagging rather than acting on it since it's outside this change.

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