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/src/index.css

@@ -20,7 +20,7 @@
   --kds-sent:     #6b7280;
 
   /* App primary — teal (shared with kitchen for recipe images etc.) */
-  --app-primary:  #0d9488;
+  --app-primary:  #ea580c; /* kds orange — distinct from kitchen's teal, D5 */
 }
 
 *, *::before, *::after {