Add Zod validation layer and AI blog generation

Introduce a schema-first validation layer and an AI blog generation
feature, plus one-command Cloudflare provisioning.

- src/schemas/ holds Zod input schemas for post, content, case,
  settings, AI, and keywords; parseForm() in src/lib/form.ts validates
  FormData and returns per-field errors.
- Migrate all admin POST handlers to parseForm, showing field-level
  errors and only redirecting once validation passes.
- Add blog_keywords table and posts.focus_keyword (migration 0002);
  uniqueSlug() centralised in src/data/content.ts.
- Add src/lib/ai.ts (OpenAI-compatible chat completions + optional
  Tavily research) and /admin/ai for AI settings, keyword queue, and
  draft generation.
- Add scripts/setup.mjs (npm run setup) to provision D1/KV, set
  secrets, and optionally migrate, seed, and deploy.
- Document AI secrets, Workers Builds deploy flow, and new schemas
  across README and AGENTS docs.

src/pages/admin/index.astro

@@ -31,6 +31,7 @@ const [caseCount] = await db.select({ n: sql<number>`count(*)` }).from(cases);
       <a class="btn secondary" href="/admin/content/faq">常見問題（{countOf("faq")}）</a>
       <a class="btn secondary" href="/admin/cases">完成案例（{caseCount?.n ?? 0}）</a>
       <a class="btn secondary" href="/admin/settings">網站設定</a>
+      <a class="btn secondary" href="/admin/ai">AI 生成</a>
     </div>
   </div>