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/schemas/expenses.py

@@ -0,0 +1,97 @@
+from pydantic import BaseModel
+from typing import Optional, List
+from datetime import datetime
+
+
+# ── Contacts ──────────────────────────────────────────────────────────────────
+
+class ContactCreate(BaseModel):
+    name: str
+    type: str = "supplier"
+    phone: Optional[str] = None
+    email: Optional[str] = None
+    address: Optional[str] = None
+    notes: Optional[str] = None
+
+
+class ContactUpdate(BaseModel):
+    name: Optional[str] = None
+    type: Optional[str] = None
+    phone: Optional[str] = None
+    email: Optional[str] = None
+    address: Optional[str] = None
+    notes: Optional[str] = None
+    is_active: Optional[bool] = None
+
+
+class ContactOut(BaseModel):
+    id: int
+    name: str
+    type: str
+    phone: Optional[str] = None
+    email: Optional[str] = None
+    address: Optional[str] = None
+    notes: Optional[str] = None
+    is_active: bool
+    created_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+# ── Expenses ──────────────────────────────────────────────────────────────────
+
+class ExpensePaymentCreate(BaseModel):
+    amount: float
+    notes: Optional[str] = None
+
+
+class ExpensePaymentOut(BaseModel):
+    id: int
+    expense_id: int
+    amount: float
+    paid_at: datetime
+    paid_by_id: int
+    paid_by_name: Optional[str] = None
+    notes: Optional[str] = None
+
+    model_config = {"from_attributes": True}
+
+
+class ExpenseCreate(BaseModel):
+    description: str
+    category: str
+    contact_id: Optional[int] = None
+    total_amount: float
+    due_date: Optional[datetime] = None
+    business_day_id: Optional[int] = None
+    notes: Optional[str] = None
+
+
+class ExpenseUpdate(BaseModel):
+    description: Optional[str] = None
+    category: Optional[str] = None
+    contact_id: Optional[int] = None
+    total_amount: Optional[float] = None
+    due_date: Optional[datetime] = None
+    notes: Optional[str] = None
+
+
+class ExpenseOut(BaseModel):
+    id: int
+    description: str
+    category: str
+    contact_id: Optional[int] = None
+    contact_name: Optional[str] = None
+    total_amount: float
+    paid_amount: float
+    due_amount: float          # computed: total - paid
+    status: str                # "paid" | "partial" | "due"
+    due_date: Optional[datetime] = None
+    business_day_id: Optional[int] = None
+    created_by_id: int
+    created_by_name: Optional[str] = None
+    created_at: datetime
+    notes: Optional[str] = None
+    payments: List[ExpensePaymentOut] = []
+
+    model_config = {"from_attributes": True}