Move seed.sql out of migrations and add SEO structured data

- Relocate seed.sql to scripts/ so wrangler d1 migrations apply
  doesn't treat it as a migration; update package.json, setup.mjs,
  README, and all AGENTS.md references.
- Add src/lib/schema-org.ts builders (LocalBusiness, FAQPage,
  BlogPosting, BreadcrumbList) and emit JSON-LD via Base.astro.
- Add lastmod to sitemap entries.
- Set real site domain, worker route, and D1/KV ids.
- Improve image loading hints and heading semantics across site
  components; switch BlogIndex to client:idle.

README.md

@@ -118,8 +118,9 @@ src/
 ├─ lib/{auth,db,env,form,media,ai,markdown}.ts
 migrations/
 ├─ 0000_init.sql …              Drizzle migrations
-└─ seed.sql                     種子資料（用 db:seed 另外執行）
-scripts/setup.mjs               一鍵 Cloudflare provision
+scripts/
+├─ setup.mjs                    一鍵 Cloudflare provision
+└─ seed.sql                     種子資料（用 db:seed 另外執行，唔可以放喺 migrations/）
 ```
 
 ## 待客戶提供（可喺 `/admin/settings` 自行更新）