Scoped DB role + own theme colour (port log E17)

- Split database.py into a runtime engine (scoped `kds` DB role, used for
  all request handling) and a migration engine (privileged `kitchen` role,
  used only at startup to create KDS's own tables and ALTER kitchen_settings)
  — KDS previously shared kitchen's full-access DB credential wholesale
- Dispose both engines on shutdown (main.py)
- Fix theme colour clash: KDS was accidentally seeded with kitchen's teal
  (#0d9488) instead of its own colour — now #ea580c (orange), regenerated
  PWA icons to match

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/vite.config.ts

@@ -14,8 +14,8 @@ export default defineConfig({
         start_url: '/kds/',
         scope: '/kds/',
         display: 'standalone',
-        theme_color: '#0d9488',
-        background_color: '#0d9488',
+        theme_color: '#ea580c',
+        background_color: '#ea580c',
         icons: [
           { src: '/kds/icons/icon-192.png', sizes: '192x192', type: 'image/png' },
           { src: '/kds/icons/icon-512.png', sizes: '512x512', type: 'image/png' },