[US-011] Add platform links (TikTok, FB, IG, Shopee)

[leimo2011]

Epic: EP-02 — Store Onboarding & Link Management \nPriority: Must Have (M) \nStory Points: 5 \nSprint: Sprint 1\n\n### User Story\nAs a store admin, I want to add links to my TikTok, Facebook, Instagram, Shopee stores so buyers can find me anywhere.\n\n### Acceptance Criteria\n- [ ] Admin can add links for platforms: TikTok, Facebook, Instagram, Shopee, YouTube, Twitter/X, Website\n- [ ] Each link has a label and URL\n- [ ] URL validation per platform format\n- [ ] Maximum links enforced per subscription tier (Free: 3, Pro/Enterprise: unlimited)\n- [ ] Links can be edited and deleted\n\n### Technical Notes\n- POST /api/stores/{id}/links/\n- PATCH /api/stores/{id}/links/{link_id}/\n- DELETE /api/stores/{id}/links/{link_id}/