feat: Phase 2I — KDS Kitchen Display Screen

Backend:
- New router /api/kds/:
  - GET /items — returns all active order items for open orders, grouped by printer zone
    (zone_id, zone_name, items array with table_name, product_name, qty, notes, added_at)
    Zones sorted by zone_id asc, no-zone column last
  - PUT /orders/{order_id}/items/{item_id}/status — mark item 'ready' (active→ready only)
    Broadcasts item_status_changed SSE event to all connected clients
- No migration needed — 'ready' is a new valid string value for order_items.status
- pay_items endpoint: now accepts 'ready' items as payable (status.in_(['active','ready']))
- active_remaining count for order status also includes 'ready' items

Frontend:
- KdsPage (/kds): full-screen dark layout, columns per printer zone
  - Zone columns: dark header with zone name + pending count, scrollable item cards
  - Item cards: table name badge, elapsed time (colour-coded: green<8m, amber<15m, red>=15m),
    product name + quantity, notes in amber, tap anywhere to mark ready
  - Optimistic marking: card dims while request in flight, disappears on success
  - SSE live updates: listens for order_updated / item_status_changed / order_paid / order_closed
  - Fallback poll every 30s; manual ↻ Ανανέωση button; live clock in top bar
  - Empty state: ✓ message when no pending items
- Sidebar: ChefHat icon for KDS (after Αποβλήτα)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

local_backend/main.py

@@ -41,6 +41,7 @@ from routers.expenses import contacts_router, expenses_router
 from routers import customers as customers_router
 from routers import tabs as tabs_router
 from routers import waste as waste_router
+from routers import kds as kds_router
 
 
 def _run_migrations():
@@ -460,3 +461,4 @@ app.include_router(expenses_router,              prefix="/api/expenses",     tag
 app.include_router(customers_router.router,      prefix="/api/customers",    tags=["customers"])
 app.include_router(tabs_router.router,           prefix="/api/tabs",         tags=["tabs"])
 app.include_router(waste_router.router,          prefix="/api/waste",        tags=["waste"])
+app.include_router(kds_router.router,            prefix="/api/kds",          tags=["kds"])