[Phase 2.4] Implement upvoting system

[Andreas-Froyland]

Goal

Let users upvote feedback items. One vote per user/session. Toggle on/off.

Depends on: #13 (detail view) - upvoting shows on both listing cards and detail page

Backend

• POST /api/feedback/[id]/vote — toggle vote
  • If not voted: insert vote, return { voted: true, count: N }
  • If already voted: remove vote, return { voted: false, count: N }
• Auth: authenticated user OR anonymous session required
• Unique constraint: one vote per (feedback_id, user_id OR session_id)
• Vote count is always computed (not cached) for consistency — optimize later if needed

Frontend

• Vote button on feedback cards (listing) and detail view
• Visual states: default (outline) vs voted (filled, accent color)
• Optimistic UI update on click, revert on error
• Show total vote count next to button
• User's own voted items are visually distinct in the listing

Notes

• Votes by anonymous sessions persist across visits (tied to session cookie)
• If anonymous user signs in, their votes carry over to their GitHub account