[US-021] Search stores by name or keyword

[leimo2011]

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 search stores by name or keyword so I can find a specific seller quickly.\n\n### Acceptance Criteria\n- [ ] Search bar prominent on home/discover screen\n- [ ] Results update as user types (debounced, 300ms)\n- [ ] Results show store name, logo, category, live badge\n- [ ] Empty state with suggestions when no results\n- [ ] Search term highlighted in results\n- [ ] Search indexed on store name, bio, category\n\n### Technical Notes\n- GET /api/stores/?search={query}\n- PostgreSQL full-text search (SearchVector) or icontains for MVP\n- Redis cache for popular search terms