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/main.py

@@ -23,6 +23,7 @@ import models.reservation   # noqa: F401
 import models.notes          # noqa: F401 — registers SiteNote, SiteTodo
 import models.expenses       # noqa: F401 — registers Contact, Expense, ExpensePayment
 import models.customers      # noqa: F401 — registers Customer
+import models.tabs           # noqa: F401 — registers Tab, TabEntry, TabPayment
 
 from routers import auth, tables, products, orders, waiters, reports, system, setup as setup_router
 from routers import business_day as business_day_router
@@ -37,6 +38,7 @@ from routers import reservations as reservations_router
 from routers import notes as notes_router
 from routers.expenses import contacts_router, expenses_router
 from routers import customers as customers_router
+from routers import tabs as tabs_router
 
 
 def _run_migrations():
@@ -275,6 +277,35 @@ def _run_migrations():
         # Phase 2B — staff payroll
         "ALTER TABLE users ADD COLUMN hourly_rate REAL",
         "ALTER TABLE waiter_shifts ADD COLUMN hourly_rate_snapshot REAL",
+        # Phase 2G — pay-later tab system
+        """CREATE TABLE IF NOT EXISTS tabs (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            customer_id INTEGER NOT NULL REFERENCES customers(id),
+            status VARCHAR NOT NULL DEFAULT 'open',
+            opened_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+            closed_at DATETIME,
+            closed_by_id INTEGER REFERENCES users(id),
+            notes TEXT
+        )""",
+        """CREATE TABLE IF NOT EXISTS tab_entries (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            tab_id INTEGER NOT NULL REFERENCES tabs(id),
+            order_id INTEGER REFERENCES orders(id),
+            order_item_id INTEGER REFERENCES order_items(id),
+            amount REAL NOT NULL,
+            description TEXT,
+            created_by_id INTEGER NOT NULL REFERENCES users(id),
+            created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+        )""",
+        """CREATE TABLE IF NOT EXISTS tab_payments (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            tab_id INTEGER NOT NULL REFERENCES tabs(id),
+            amount REAL NOT NULL,
+            payment_method VARCHAR,
+            received_by_id INTEGER NOT NULL REFERENCES users(id),
+            notes TEXT,
+            created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+        )""",
         # Phase 2F — customer CRM
         """CREATE TABLE IF NOT EXISTS customers (
             id INTEGER PRIMARY KEY AUTOINCREMENT,
@@ -412,3 +443,4 @@ app.include_router(notes_router.router,          prefix="/api/notes",        tag
 app.include_router(contacts_router,              prefix="/api/contacts",     tags=["contacts"])
 app.include_router(expenses_router,              prefix="/api/expenses",     tags=["expenses"])
 app.include_router(customers_router.router,      prefix="/api/customers",    tags=["customers"])
+app.include_router(tabs_router.router,           prefix="/api/tabs",         tags=["tabs"])