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/meta/_journal.json

@@ -15,6 +15,13 @@
       "when": 1789107074198,
       "tag": "0001_regular_multiple_man",
       "breakpoints": true
+    },
+    {
+      "idx": 2,
+      "version": "6",
+      "when": 1789114640162,
+      "tag": "0002_red_roughhouse",
+      "breakpoints": true
     }
   ]
 }