feat: Phase 2D — expense tracking + supplier contact book

Backend:
- New models: Contact, Expense, ExpensePayment (3 new tables)
- Expense status (paid/partial/due) is always computed from paid_amount vs total_amount — never stored
- paid_amount recomputed from sum of all payments on each payment write (no client trust)
- New router /api/contacts/: list, create, update, soft-delete (is_active=false)
- New router /api/expenses/: list (filter by status/category/contact), create, update, delete,
  record payment, summary (total_due, total_paid, by_category)
- Migrations: CREATE TABLE IF NOT EXISTS for contacts, expenses, expense_payments

Frontend:
- ContactsPage (/contacts): searchable table, type badge (Προμηθευτής/Προσωπικό/Κοινή Χρεία/Άλλο),
  create/edit modal, soft-delete
- ExpensesPage (/expenses): filter bar (status + category), expandable rows with payment history,
  summary header showing total outstanding, inline Payment modal (defaults to full due amount),
  create/edit expense modal
- Sidebar: Receipt icon for Έξοδα, BookUser icon for Επαφές

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

local_backend/main.py

@@ -21,6 +21,7 @@ import models.flag          # noqa: F401 — registers TableFlagDef, TableFlagAs
 import models.message       # noqa: F401 — registers StaffMessage, StaffMessageAck, QuickMessageTemplate
 import models.reservation   # noqa: F401
 import models.notes          # noqa: F401 — registers SiteNote, SiteTodo
+import models.expenses       # noqa: F401 — registers Contact, Expense, ExpensePayment
 
 from routers import auth, tables, products, orders, waiters, reports, system, setup as setup_router
 from routers import business_day as business_day_router
@@ -33,6 +34,7 @@ from routers import data_transfer as data_transfer_router
 from routers import connect_orders as connect_orders_router
 from routers import reservations as reservations_router
 from routers import notes as notes_router
+from routers.expenses import contacts_router, expenses_router
 
 
 def _run_migrations():
@@ -271,6 +273,39 @@ 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 2D — expense tracking + supplier contacts
+        """CREATE TABLE IF NOT EXISTS contacts (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            name VARCHAR NOT NULL,
+            type VARCHAR NOT NULL DEFAULT 'supplier',
+            phone VARCHAR,
+            email VARCHAR,
+            address TEXT,
+            notes TEXT,
+            is_active INTEGER NOT NULL DEFAULT 1,
+            created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+        )""",
+        """CREATE TABLE IF NOT EXISTS expenses (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            description VARCHAR NOT NULL,
+            category VARCHAR NOT NULL,
+            contact_id INTEGER REFERENCES contacts(id),
+            total_amount REAL NOT NULL,
+            paid_amount REAL NOT NULL DEFAULT 0.0,
+            due_date DATETIME,
+            business_day_id INTEGER REFERENCES business_days(id),
+            created_by_id INTEGER NOT NULL REFERENCES users(id),
+            created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+            notes TEXT
+        )""",
+        """CREATE TABLE IF NOT EXISTS expense_payments (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            expense_id INTEGER NOT NULL REFERENCES expenses(id),
+            amount REAL NOT NULL,
+            paid_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+            paid_by_id INTEGER NOT NULL REFERENCES users(id),
+            notes TEXT
+        )""",
         # Phase 2C — notes & todos
         """CREATE TABLE IF NOT EXISTS site_notes (
             id INTEGER PRIMARY KEY AUTOINCREMENT,
@@ -353,3 +388,5 @@ app.include_router(data_transfer_router.router,  prefix="/api/data-transfer", ta
 app.include_router(connect_orders_router.router, prefix="/api/connect",      tags=["connect"])
 app.include_router(reservations_router.router,   prefix="/api/reservations", tags=["reservations"])
 app.include_router(notes_router.router,          prefix="/api/notes",        tags=["notes"])
+app.include_router(contacts_router,              prefix="/api/contacts",     tags=["contacts"])
+app.include_router(expenses_router,              prefix="/api/expenses",     tags=["expenses"])