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.

package.json

@@ -19,7 +19,8 @@
     "db:studio": "drizzle-kit studio",
     "kv:create": "wrangler kv namespace create CACHE",
     "secret": "wrangler secret put ADMIN_PASSWORD",
-    "types": "wrangler types"
+    "types": "wrangler types",
+    "setup": "node scripts/setup.mjs"
   },
   "dependencies": {
     "@astrojs/cloudflare": "^14.3.1",
@@ -33,7 +34,8 @@
     "lucide-react": "^1.44.0",
     "marked": "^18.0.12",
     "react": "^19.3.0",
-    "react-dom": "^19.3.0"
+    "react-dom": "^19.3.0",
+    "zod": "^4.6.2"
   },
   "devDependencies": {
     "@types/node": "^22.20.2",