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/schemas/customers.py

@@ -0,0 +1,41 @@
+from pydantic import BaseModel
+from typing import Optional, List
+from datetime import datetime
+
+
+class CustomerCreate(BaseModel):
+    name: str
+    nickname: Optional[str] = None
+    phone: Optional[str] = None
+    email: Optional[str] = None
+    notes: Optional[str] = None
+
+
+class CustomerUpdate(BaseModel):
+    name: Optional[str] = None
+    nickname: Optional[str] = None
+    phone: Optional[str] = None
+    email: Optional[str] = None
+    notes: Optional[str] = None
+    is_active: Optional[bool] = None
+
+
+class CustomerOut(BaseModel):
+    id: int
+    name: str
+    nickname: Optional[str] = None
+    phone: Optional[str] = None
+    email: Optional[str] = None
+    notes: Optional[str] = None
+    is_active: bool
+    created_at: datetime
+    created_by_id: int
+    # computed stats (set by router)
+    visit_count: int = 0
+    total_spent: float = 0.0
+
+    model_config = {"from_attributes": True}
+
+
+class AssignCustomerRequest(BaseModel):
+    customer_id: Optional[int] = None  # null = unassign