feat: Phase 2F — customer CRM

Backend:
- New model: Customer (customers table) — name, nickname, phone, email, notes, is_active
- Order model: add customer_id nullable FK → customers
- New router: /api/customers/ — list (with ?search=), create, get, update, soft-delete,
  GET /{id}/orders (visit history with totals)
- Each CustomerOut includes visit_count + total_spent (computed from closed/paid orders)
- PUT /api/orders/{id}/customer — assign or unassign a customer; manager-only;
  validates customer is active; broadcasts order_updated SSE event
- GET /api/orders/{id} now returns customer_id, customer_name (with nickname), customer_phone
- Migration: CREATE TABLE customers + ALTER TABLE orders ADD COLUMN customer_id

Frontend:
- CustomersPage (/customers): searchable list (name/nickname/phone), avatar initials,
  visit count + total_spent on each row; click → detail panel slides in showing stats
  (visits, total spent, avg ticket), contact info, notes, full visit history
- OrderDetailPage: new Πελάτης card — shows assigned customer in blue if set;
  for open orders shows "+ Ανάθεση πελάτη" button → inline search dropdown → assign;
  Αφαίρεση button to unassign
- Sidebar: Users icon for Πελάτες (between Επαφές and Ρυθμίσεις)

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

local_backend/models/order.py

@@ -30,10 +30,13 @@ class Order(Base):
     online_customer_address = Column(Text, nullable=True)
     online_customer_notes = Column(Text, nullable=True)
     online_order_type = Column(String, nullable=True)               # "delivery" | "dine_in"
+    # Phase 2F — customer CRM
+    customer_id = Column(Integer, ForeignKey("customers.id"), nullable=True)
 
     table = relationship("Table", back_populates="orders")
     opener = relationship("User", foreign_keys=[opened_by], back_populates="orders_opened")
     closer = relationship("User", foreign_keys=[closed_by], back_populates="orders_closed")
+    customer = relationship("Customer", back_populates="orders", foreign_keys=[customer_id])
     items = relationship("OrderItem", back_populates="order", cascade="all, delete-orphan")
     waiters = relationship("OrderWaiter", back_populates="order", cascade="all, delete-orphan")
     print_logs = relationship("PrintLog", back_populates="order", cascade="all, delete-orphan")