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/models/expenses.py

@@ -0,0 +1,58 @@
+from sqlalchemy import Column, Integer, String, Boolean, DateTime, Float, ForeignKey, Text
+from sqlalchemy.orm import relationship
+from datetime import datetime, timezone
+from database import Base
+
+
+def _utcnow():
+    return datetime.now(timezone.utc)
+
+
+class Contact(Base):
+    __tablename__ = "contacts"
+
+    id = Column(Integer, primary_key=True, index=True)
+    name = Column(String, nullable=False)
+    type = Column(String, default="supplier", nullable=False)  # supplier|staff|utility|other
+    phone = Column(String, nullable=True)
+    email = Column(String, nullable=True)
+    address = Column(Text, nullable=True)
+    notes = Column(Text, nullable=True)
+    is_active = Column(Boolean, default=True, nullable=False)
+    created_at = Column(DateTime(timezone=True), default=_utcnow)
+
+    expenses = relationship("Expense", back_populates="contact")
+
+
+class Expense(Base):
+    __tablename__ = "expenses"
+
+    id = Column(Integer, primary_key=True, index=True)
+    description = Column(String, nullable=False)
+    category = Column(String, nullable=False)  # supplier|utilities|rent|maintenance|staff|equipment|other
+    contact_id = Column(Integer, ForeignKey("contacts.id"), nullable=True)
+    total_amount = Column(Float, nullable=False)
+    paid_amount = Column(Float, default=0.0, nullable=False)
+    due_date = Column(DateTime(timezone=True), nullable=True)
+    business_day_id = Column(Integer, ForeignKey("business_days.id"), nullable=True)
+    created_by_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+    created_at = Column(DateTime(timezone=True), default=_utcnow)
+    notes = Column(Text, nullable=True)
+
+    contact = relationship("Contact", back_populates="expenses")
+    created_by = relationship("User", foreign_keys=[created_by_id])
+    payments = relationship("ExpensePayment", back_populates="expense", cascade="all, delete-orphan")
+
+
+class ExpensePayment(Base):
+    __tablename__ = "expense_payments"
+
+    id = Column(Integer, primary_key=True, index=True)
+    expense_id = Column(Integer, ForeignKey("expenses.id"), nullable=False)
+    amount = Column(Float, nullable=False)
+    paid_at = Column(DateTime(timezone=True), default=_utcnow)
+    paid_by_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+    notes = Column(Text, nullable=True)
+
+    expense = relationship("Expense", back_populates="payments")
+    paid_by = relationship("User", foreign_keys=[paid_by_id])