feat: add delete post to Agent Plaza (admin + author)

[Longado]

Summary

Agent Plaza currently has no way to remove posts. This adds a delete feature with proper permission checks.

Changes

Backend — backend/app/api/plaza.py

• New DELETE /api/plaza/posts/{post_id} endpoint
• platform_admin and org_admin can delete any post
• Post author can delete their own post
• Returns 403 if neither condition is met
• Cascade delete of comments/likes is handled by the existing SQLAlchemy PlazaPost relationship

Frontend — frontend/src/pages/Plaza.tsx

• deletePost mutation using authenticated fetch with Bearer token
• 🗑 delete button appears on posts only for admins or the post's own author
• Confirm dialog before deletion to prevent accidental removal
• Button is unobtrusive by default (muted, 60% opacity), turns red on hover
• Invalidates plaza-posts and plaza-stats queries on success so counts update immediately

Test plan

• Admin (platform_admin) can delete any post
• Post author can delete their own post
• Regular member cannot see delete button on others' posts
• Confirm dialog prevents accidental deletion
• Post list and stats refresh immediately after deletion

🤖 Generated with Claude Code

[wisdomqin]

Thanks for this contribution! The feature logic and permission model are solid. A few things to address before merging:

Backend

1. Add audit logging — Delete is a destructive action. Consider adding a logger.info line recording who deleted which post, especially for admin deletions. Example:

   logger.info(f"Plaza post {post_id} deleted by user {current_user.id} (admin={is_admin})")

Frontend

2. Replace emoji with SVG icon — The 🗑 emoji is inconsistent with the rest of the UI which uses SVG icons (Icons.heart, Icons.comment). Please add a trash icon to the Icons object and use it here.

3. Replace window.confirm with a styled dialog — The native browser confirm dialog breaks the visual consistency of the app (we follow a Linear-style dark theme). Please use a custom confirmation modal or at minimum a styled dialog component.

4. Move inline hover logic to CSS — The onMouseEnter/onMouseLeave pattern for hover effects is fragile. Use a CSS class with :hover pseudo-selector instead:

   .delete-btn { opacity: 0.6; color: var(--text-muted); }
   .delete-btn:hover { opacity: 1; color: #ef4444; }

5. Add i18n support — The project uses react-i18next. Hardcoded strings like "Delete this post?" and "Delete post" should use translation keys (e.g. t('plaza.deleteConfirm')).

Please address these items and I'll be happy to approve. The backend logic itself looks great!