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.

migrations/seed.sql

@@ -21,6 +21,15 @@ INSERT OR REPLACE INTO site_settings (key, value, updated_at) VALUES
   ('seo_default_description', '專營香港村屋太陽能系統：現場評估、合法支架、代辦申請、專業安裝及驗收認證。透明報價，專人跟進，幫你善用天台賺取發電回報。', strftime('%s','now')*1000),
   ('og_image', 'https://images.unsplash.com/photo-1509391366360-2e959784a276?auto=format&fit=crop&w=1200&q=80', strftime('%s','now')*1000);
 
+INSERT OR REPLACE INTO site_settings (key, value, updated_at) VALUES
+  ('ai_enabled', '1', strftime('%s','now')*1000),
+  ('ai_base_url', 'https://api.deepinfra.com/v1/openai', strftime('%s','now')*1000),
+  ('ai_chat_model', 'deepseek-ai/DeepSeek-V3-0324', strftime('%s','now')*1000),
+  ('ai_context_prompt', '用香港繁體中文（廣東話書面語）書寫，語氣專業、親切、務實，避免誇大。', strftime('%s','now')*1000),
+  ('ai_business_context', '盈豐太陽能工程有限公司專營香港村屋太陽能系統：現場評估、合法鋁合金支架、代辦中電／港燈上網電價（FiT）申請、專業安裝及驗收認證。着重透明報價、專人跟進、安全合規。', strftime('%s','now')*1000),
+  ('ai_web_search_enabled', '0', strftime('%s','now')*1000),
+  ('ai_web_search_max_results', '5', strftime('%s','now')*1000);
+
 INSERT OR REPLACE INTO content_items (id, kind, title, description, extra, sort_order, status, updated_at) VALUES
   ('svc-001', 'service', '村屋天台太陽能系統', '善用村屋天台空間，安裝高效太陽能板，將閒置天台變成穩定收入來源。', 'sun', 1, 'published', strftime('%s','now')*1000),
   ('svc-002', 'service', '合法合規支架工程', '按村屋結構同法規設計鋁合金支架，穩固耐用，確保合乎相關要求。', 'shield', 2, 'published', strftime('%s','now')*1000),
@@ -54,6 +63,11 @@ INSERT OR REPLACE INTO cases (id, title, location, completed_at, description, im
   ('case-002', '元朗山下村 — 香檳色鋁合金支架', '元朗屏山', '2025年7月', '香檳色支架配合村屋外牆色調，施工整齊，發電穩定。', 'https://images.unsplash.com/photo-1592833159155-c62df1b65634?auto=format&fit=crop&w=1200&q=80', 2, 'published', strftime('%s','now')*1000),
   ('case-003', '西貢西澳村 — 焗白色鋁合金支架', '新界西貢', '2025年5月', '焗白色支架低調美觀，善用天台每一寸可用空間。', 'https://images.unsplash.com/photo-1508514177221-188b1cf16e9d?auto=format&fit=crop&w=1200&q=80', 3, 'published', strftime('%s','now')*1000);
 
+INSERT OR REPLACE INTO blog_keywords (id, keyword, status, created_at, used_at, post_id) VALUES
+  ('kw-001', '村屋太陽能回本期', 'pending', strftime('%s','now')*1000, NULL, NULL),
+  ('kw-002', '村屋天台安裝太陽能注意事項', 'pending', strftime('%s','now')*1000, NULL, NULL),
+  ('kw-003', '上網電價 FiT 申請流程', 'pending', strftime('%s','now')*1000, NULL, NULL);
+
 INSERT OR REPLACE INTO posts (id, slug, title, excerpt, content, cover_image, tags, meta_description, status, published_at, updated_at) VALUES
   ('post-001', 'village-house-solar-guide', '村屋安裝太陽能前，你要知嘅 5 件事', '想喺村屋天台裝太陽能？由天台面積、日照、支架法規到回本期，呢篇幫你一次過搞清楚。',
 '村屋天台係香港少數可以合法大規模安裝太陽能嘅地方。不過落決定之前，以下 5 點你一定要知。