feat: migrate website-creator from Next.js+Payload to Astro+Tina CMS

Major changes:
- Replace Payload CMS with Tina CMS (self-hosted)
- Add Astro DB for consent logging (PDPA compliant)
- Update Tailwind v3 to v4 (@tailwindcss/vite plugin)
- Add astro-tina-starter template
- Rewrite consent template for Astro (ConsentBanner.astro, Astro DB, Nano Stores)
- Add install-tina-backend.sh for self-hosted Tina per customer
- Rename convert-astro.sh to migrate-tina.sh
- Add AGENTS.md template for generated websites
- Delete all Payload/Next.js files

Technical updates:
- Astro DB using defineDb with eq operators for queries
- Tailwind v4 with @theme block
- Tina CMS local development mode
- Proper Astro API routes for consent

Research-verified with official documentation (April 2026)

skills/website-creator/templates/astro-tina-starter/.tina/config.ts

@@ -0,0 +1,18 @@
+import { defineConfig } from 'tinacms';
+import { schema } from './schema';
+
+export default defineConfig({
+  schema,
+  ui: {
+    navigation: {
+      'content/posts': { label: 'Posts' },
+      'content/pages': { label: 'Pages' },
+    },
+  },
+  media: {
+    tina: {
+      publicFolder: 'public',
+      mediaRoot: 'uploads',
+    },
+  },
+});