[US-025] Follow favorite stores #37
Description
Epic: EP-04 — End User Discovery \nPriority: Must Have (M) \nStory Points: 5 \nSprint: Sprint 2\n\n### User Story\nAs an end user, I want to follow my favorite stores so I get notified when they go live.\n\n### Acceptance Criteria\n- [ ] Follow/unfollow button on store profile\n- [ ] Follower count shown on store card\n- [ ] User's followed stores accessible in 'My Stores' tab\n- [ ] Push notification sent via FCM when followed store goes live\n- [ ] Follow limited to logged-in users only\n\n### Technical Notes\n- POST /api/stores/{id}/follow/\n- DELETE /api/stores/{id}/follow/\n- Celery task triggers FCM push to all followers when is_live flips to true