feat: Phase 2G — pay-later tab system

Backend:
- New models: Tab, TabEntry, TabPayment (3 new tables)
  - Tab: open|closed|forgiven, one open tab per customer enforced
  - TabEntry: what went ON the tab (per-item snapshot with description)
  - TabPayment: what came OFF the tab; balance = sum(entries) - sum(payments), never stored
- New router /api/tabs/:
  - GET / — list open tabs (sorted by balance desc); ?tab_status= override
  - GET /{id} — full tab detail with entries + payments
  - POST / — open new tab for a customer (400 if one already exists)
  - POST /{id}/entries — add entry directly (amount + description)
  - POST /{id}/pay — record payment; validates amount <= balance
  - POST /{id}/close — close tab (requires balance = 0)
  - POST /{id}/forgive — write off remaining balance
  - GET /customer/{customer_id} — all tabs for a customer (open first)
- POST /api/orders/{id}/items/{item_id}/tab:
  - Requires order has a customer assigned
  - Finds or auto-creates open tab for that customer
  - Sets order_item.status = "tabbed" (new valid status value)
  - Creates TabEntry with auto-generated description
  - Broadcasts order_updated SSE event
- Migrations: CREATE TABLE IF NOT EXISTS for tabs, tab_entries, tab_payments

Frontend:
- TabsPage (/tabs): open tabs list sorted by balance; each card shows charges/payments
  history, live balance, Πληρωμή modal (defaults to full balance), Κλείσιμο button
  (only shown when balance=0), Χάρισμα Υπολοίπου with confirm modal + reason
- CustomersPage CustomerDetail: open tab banner showing balance + entry count
  (appears between stats and contact info when customer has an open tab)
- OrderDetailPage: 📋 Καρτέλα button appears on active items when order has a customer;
  tabbed items show a 📋 Καρτέλα badge; tabItem mutation calls /items/{id}/tab
- Sidebar: CreditCard icon for Καρτέλες (after Πελάτες)

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

local_backend/routers/orders.py

@@ -583,6 +583,10 @@ class AssignCustomerBody(BaseModel):
     customer_id: Optional[int] = None  # null = unassign
 
 
+class TabItemBody(BaseModel):
+    pass  # no body needed — tab derived from order's customer
+
+
 @router.put("/{order_id}/customer")
 def assign_customer(order_id: int, body: AssignCustomerBody, db: Session = Depends(get_db), user: User = Depends(require_manager)):
     from models.customers import Customer
@@ -958,3 +962,55 @@ def print_synopsis(
 
     background_tasks.add_task(print_order_synopsis, printer.ip_address, printer.port, synopsis, printer.line_width)
     return {"status": "printing"}
+
+
+# ─── Phase 2G: Put item on customer tab ───────────────────────────────────────
+
+@router.post("/{order_id}/items/{item_id}/tab")
+def put_item_on_tab(
+    order_id: int,
+    item_id: int,
+    db: Session = Depends(get_db),
+    user: User = Depends(require_manager),
+):
+    from models.tabs import Tab, TabEntry
+
+    order = db.query(Order).filter(Order.id == order_id).first()
+    if not order:
+        raise HTTPException(status_code=404, detail="Order not found")
+    if not order.customer_id:
+        raise HTTPException(status_code=400, detail="Order has no customer assigned — assign a customer first")
+
+    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 already {item.status} — cannot tab it")
+
+    # Find or auto-create the customer's open tab
+    tab = db.query(Tab).filter(Tab.customer_id == order.customer_id, Tab.status == "open").first()
+    if not tab:
+        tab = Tab(customer_id=order.customer_id)
+        db.add(tab)
+        db.flush()
+
+    # Build description
+    product_name = item.product.name if item.product else f"#{item.product_id}"
+    description = f"{product_name} ×{item.quantity} @ €{item.unit_price:.2f}"
+
+    entry = TabEntry(
+        tab_id=tab.id,
+        order_id=order_id,
+        order_item_id=item_id,
+        amount=round(item.unit_price * item.quantity, 2),
+        description=description,
+        created_by_id=user.id,
+    )
+    db.add(entry)
+
+    # Mark item as tabbed — distinct from active/paid/cancelled
+    item.status = "tabbed"
+
+    db.commit()
+    broadcast_sync("order_updated", {"order_id": order.id, "table_id": order.table_id, "status": order.status, "action": "item_tabbed"})
+    return {"status": "tabbed", "tab_id": tab.id, "entry_amount": entry.amount}