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/routers/kds.py

@@ -0,0 +1,128 @@
+"""
+KDS — Kitchen Display System router.
+
+GET  /api/kds/items
+    Returns all active (status='active') order items for open orders,
+    enriched with zone name and table name. Used by the KDS frontend.
+
+PUT  /api/orders/{order_id}/items/{item_id}/status
+    Mark an item ready (active → ready). Only that transition is allowed here.
+    Broadcasts item_status_changed SSE event.
+"""
+from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy.orm import Session
+from pydantic import BaseModel
+from datetime import datetime, timezone
+
+from database import get_db
+from models.order import Order, OrderItem
+from models.product import Product
+from models.printer import Printer
+from models.table import Table
+from models.user import User
+from routers.deps import get_current_user
+from services.sse_bus import broadcast_sync
+
+router = APIRouter()
+
+
+class ItemStatusUpdate(BaseModel):
+    status: str  # only "ready" is accepted
+
+
+@router.get("/items")
+def kds_items(
+    db: Session = Depends(get_db),
+    user: User = Depends(get_current_user),
+):
+    """Return all active order items grouped by zone for the KDS display."""
+    # Only look at open / partially_paid orders
+    open_orders = db.query(Order).filter(Order.status.in_(["open", "partially_paid"])).all()
+    order_ids = [o.id for o in open_orders]
+    if not order_ids:
+        return {"zones": []}
+
+    order_map = {o.id: o for o in open_orders}
+
+    items = db.query(OrderItem).filter(
+        OrderItem.order_id.in_(order_ids),
+        OrderItem.status == "active",
+    ).order_by(OrderItem.added_at.asc()).all()
+
+    tables_map = {t.id: (t.label or f"T{t.number}") for t in db.query(Table).all()}
+    printers_map = {p.id: p.name for p in db.query(Printer).all()}
+
+    # Zone = printer_zone_id (None = no zone)
+    zones: dict = {}
+
+    def _zone_key(zone_id):
+        if zone_id is None:
+            return 0
+        return zone_id
+
+    def _zone_name(zone_id):
+        if zone_id is None:
+            return "Χωρίς Ζώνη"
+        return printers_map.get(zone_id, f"Ζώνη #{zone_id}")
+
+    for item in items:
+        product = item.product
+        zone_id = product.printer_zone_id if product else None
+        zkey = _zone_key(zone_id)
+
+        if zkey not in zones:
+            zones[zkey] = {
+                "zone_id": zone_id,
+                "zone_name": _zone_name(zone_id),
+                "items": [],
+            }
+
+        order = order_map.get(item.order_id)
+        table_name = tables_map.get(order.table_id) if order and order.table_id else None
+
+        zones[zkey]["items"].append({
+            "id": item.id,
+            "order_id": item.order_id,
+            "table_name": table_name,
+            "product_name": product.name if product else f"#{item.product_id}",
+            "quantity": item.quantity,
+            "notes": item.notes,
+            "added_at": item.added_at,
+            "status": item.status,
+        })
+
+    # Sort zones: named zones first (by zone_id), then no-zone last
+    zone_list = sorted(zones.values(), key=lambda z: (z["zone_id"] is None, z["zone_id"] or 0))
+    return {"zones": zone_list}
+
+
+@router.put("/orders/{order_id}/items/{item_id}/status")
+def update_item_status(
+    order_id: int,
+    item_id: int,
+    body: ItemStatusUpdate,
+    db: Session = Depends(get_db),
+    user: User = Depends(get_current_user),
+):
+    if body.status != "ready":
+        raise HTTPException(status_code=400, detail="Only 'ready' is a valid status transition via this endpoint")
+
+    item = db.query(OrderItem).filter(
+        OrderItem.id == item_id,
+        OrderItem.order_id == order_id,
+    ).first()
+    if not item:
+        raise HTTPException(status_code=404, detail="Item not found")
+    if item.status != "active":
+        raise HTTPException(status_code=400, detail=f"Item is '{item.status}' — only active items can be marked ready")
+
+    item.status = "ready"
+    db.commit()
+
+    broadcast_sync("item_status_changed", {
+        "order_id": order_id,
+        "item_id": item_id,
+        "status": "ready",
+    })
+
+    return {"status": "ready", "item_id": item_id}