feat: Add auto-approve toggle buttons for posts, comments & teacher reviews

- Added 3 toggle switches in Admin Settings (Auto Approve section):
  - Auto Approve Posts (default: OFF)
  - Auto Approve Comments (default: ON for backward compat)
  - Auto Approve Teacher Reviews (default: OFF)

- PostController: already had auto-approve logic, verified working
- Comment model: added pending status support with Setting check
  - When disabled: comments go to 'pending', don't increment count
  - When approved by admin: count incremented correctly
  - Added findByIdAny(), getModerationQueue(), countByStatus(),
    approve(), reject() moderation methods
- TeacherReview model: checks auto_approve_reviews setting
  - Approved reviews show 'published' message, pending show 'waiting'

- Created CommentModerationController for admin comment moderation
- Created admin/comments.php view with pending/active/rejected tabs
- Added Comments link to admin sidebar with pending badge counter
- Added comment moderation routes (approve/reject/delete)
- Frontend feed.js handles pending comments gracefully (info toast)
