Add Mago4 demo ERP with Python FastAPI

Implements a working in-memory ERP demo inspired by Mago4, covering
the core modules: dashboard with KPI widgets, customers, suppliers,
products, sales orders, purchase orders, and warehouse inventory.

- FastAPI backend with Pydantic models and REST API (auto-docs at /docs)
- Jinja2 HTML templates with Mago4-style UI (dark sidebar, blue/orange/
  green/red palette, Material Design icons)
- In-memory fake database seeded with sample customers, suppliers,
  products, and orders — ready to swap for SQLAlchemy + SQL Server
- Workarounds for Python 3.14 + Starlette 1.3 / Jinja2 compatibility
  (cache_size=0, new TemplateResponse(request, name, ctx) signature)
- launch.json for one-click preview; run.bat for manual startup

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

models.py

@@ -0,0 +1,193 @@
+from pydantic import BaseModel
+from typing import Optional, List
+from datetime import datetime
+from enum import Enum
+
+
+class OrderStatus(str, Enum):
+    DRAFT = "draft"
+    CONFIRMED = "confirmed"
+    PARTIALLY_RECEIVED = "partially_received"
+    RECEIVED = "received"
+    CANCELLED = "cancelled"
+
+
+class OrderType(str, Enum):
+    SALES = "sales"
+    PURCHASE = "purchase"
+
+
+class Customer(BaseModel):
+    id: int
+    code: str
+    name: str
+    email: str
+    phone: str
+    address: str
+    city: str
+    country: str
+    credit_limit: float
+    balance: float
+
+    class Config:
+        json_schema_extra = {
+            "example": {
+                "id": 1,
+                "code": "CUST001",
+                "name": "Acme Corp",
+                "email": "info@acme.com",
+                "phone": "+39 02 1234567",
+                "address": "Via Roma 10",
+                "city": "Milano",
+                "country": "IT",
+                "credit_limit": 50000,
+                "balance": 15000
+            }
+        }
+
+
+class Supplier(BaseModel):
+    id: int
+    code: str
+    name: str
+    email: str
+    phone: str
+    address: str
+    city: str
+    country: str
+    payment_terms_days: int
+    balance: float
+
+    class Config:
+        json_schema_extra = {
+            "example": {
+                "id": 1,
+                "code": "SUPP001",
+                "name": "Global Components Ltd",
+                "email": "sales@globalcomp.com",
+                "phone": "+44 20 7946 0958",
+                "address": "123 Oxford Street",
+                "city": "London",
+                "country": "UK",
+                "payment_terms_days": 30,
+                "balance": 5000
+            }
+        }
+
+
+class Product(BaseModel):
+    id: int
+    code: str
+    description: str
+    category: str
+    unit_price: float
+    quantity_in_stock: int
+    reorder_level: int
+    supplier_id: Optional[int] = None
+
+    class Config:
+        json_schema_extra = {
+            "example": {
+                "id": 1,
+                "code": "PROD001",
+                "description": "Industrial Widget A",
+                "category": "Electronics",
+                "unit_price": 125.50,
+                "quantity_in_stock": 250,
+                "reorder_level": 50,
+                "supplier_id": 1
+            }
+        }
+
+
+class OrderLine(BaseModel):
+    id: int
+    product_id: int
+    product_code: str
+    product_description: str
+    quantity: float
+    unit_price: float
+    total_amount: float
+
+    class Config:
+        json_schema_extra = {
+            "example": {
+                "id": 1,
+                "product_id": 1,
+                "product_code": "PROD001",
+                "product_description": "Industrial Widget A",
+                "quantity": 10,
+                "unit_price": 125.50,
+                "total_amount": 1255.00
+            }
+        }
+
+
+class SalesOrder(BaseModel):
+    id: int
+    order_number: str
+    order_date: datetime
+    customer_id: int
+    customer_name: str
+    status: OrderStatus
+    lines: List[OrderLine]
+    total_amount: float
+    delivery_date: Optional[datetime] = None
+
+    class Config:
+        json_schema_extra = {
+            "example": {
+                "id": 1,
+                "order_number": "SO-2024-001",
+                "order_date": "2024-01-15T10:30:00",
+                "customer_id": 1,
+                "customer_name": "Acme Corp",
+                "status": "confirmed",
+                "lines": [],
+                "total_amount": 2500.00,
+                "delivery_date": "2024-02-15T00:00:00"
+            }
+        }
+
+
+class PurchaseOrder(BaseModel):
+    id: int
+    order_number: str
+    order_date: datetime
+    supplier_id: int
+    supplier_name: str
+    status: OrderStatus
+    lines: List[OrderLine]
+    total_amount: float
+    expected_delivery_date: Optional[datetime] = None
+
+    class Config:
+        json_schema_extra = {
+            "example": {
+                "id": 1,
+                "order_number": "PO-2024-001",
+                "order_date": "2024-01-10T14:00:00",
+                "supplier_id": 1,
+                "supplier_name": "Global Components Ltd",
+                "status": "confirmed",
+                "lines": [],
+                "total_amount": 5000.00,
+                "expected_delivery_date": "2024-02-10T00:00:00"
+            }
+        }
+
+
+class DashboardWidget(BaseModel):
+    title: str
+    subtitle: Optional[str] = None
+    style: str  # "stats", "chart", "grid"
+    size: str = "small"  # "small", "medium", "large"
+    icon: Optional[str] = None
+    color: Optional[str] = None
+    value: Optional[str] = None
+    format: Optional[str] = None  # "money", "number", "date"
+    data: Optional[dict] = None
+
+
+class Dashboard(BaseModel):
+    widgets: List[List[DashboardWidget]]