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>

manager_dashboard/src/pages/CustomersPage.jsx

@@ -98,6 +98,13 @@ function CustomerDetail({ customer, onEdit, onClose }) {
     staleTime: 30_000,
   })
 
+  const { data: tabsData = [] } = useQuery({
+    queryKey: ['customer-tabs', customer.id],
+    queryFn: () => client.get(`/api/tabs/customer/${customer.id}`).then(r => r.data),
+    staleTime: 30_000,
+  })
+
+  const openTab = tabsData.find(t => t.status === 'open')
   const orders = data?.orders || []
 
   return (
@@ -128,6 +135,21 @@ function CustomerDetail({ customer, onEdit, onClose }) {
         ))}
       </div>
 
+      {/* Open tab banner */}
+      {openTab && (
+        <div style={{ padding: '10px 22px', borderBottom: '1px solid #f0f0ef', flexShrink: 0 }}>
+          <div style={{ display: 'flex', alignItems: 'center', justifyContent: 'space-between', padding: '10px 14px', background: '#fef2f2', border: '1px solid #fecaca', borderRadius: 10 }}>
+            <div>
+              <div style={{ fontSize: 12.5, fontWeight: 700, color: '#dc2626' }}>Ανοιχτή Καρτέλα</div>
+              <div style={{ fontSize: 12, color: '#9ca3af', marginTop: 1 }}>
+                {openTab.entries.length} χρεώσεις · πληρωμένο {fmt(openTab.total_paid)}
+              </div>
+            </div>
+            <div style={{ fontSize: 18, fontWeight: 800, color: '#dc2626' }}>{fmt(openTab.balance)}</div>
+          </div>
+        </div>
+      )}
+
       {/* Contact info */}
       <div style={{ padding: '12px 22px', borderBottom: '1px solid #f0f0ef', flexShrink: 0 }}>
         {(customer.phone || customer.email || customer.notes) ? (