feat(connect): Phase 1 — add digital product fields to local_backend

Adds seven digital-menu-specific columns to the products table so
managers can control the public Xenia Connect menu independently of POS
data:

  - digital_visible   (INTEGER, default 1) — hide product from menu
  - digital_available (INTEGER, default 1) — show as out-of-stock
  - digital_name      (VARCHAR, nullable)  — override display name
  - digital_description (VARCHAR, nullable)
  - digital_price     (REAL, nullable)     — override price
  - digital_discount  (REAL, default 0.0)  — percentage discount
  - digital_image_url (VARCHAR, nullable)  — menu-specific image

Changes:
  - main.py: 7 additive ALTER TABLE migrations (safe on existing DBs)
  - models/product.py: matching SQLAlchemy columns on Product
  - schemas/product.py: ProductBase carries defaults (visible/available
    default True); ProductUpdate has all fields Optional; ProductOut
    inherits everything via ProductBase

No router changes needed — PUT /api/products/{id} picks up the new
fields from ProductUpdate automatically.

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

local_backend/schemas/product.py

@@ -263,6 +263,14 @@ class ProductBase(BaseModel):
     lifecycle_status: str = "active"
     printer_zone_id: Optional[int] = None
     sort_order: int = 0
+    # Xenia Connect — digital menu overrides
+    digital_visible: bool = True
+    digital_available: bool = True
+    digital_name: Optional[str] = None
+    digital_description: Optional[str] = None
+    digital_price: Optional[float] = None
+    digital_discount: float = 0.0
+    digital_image_url: Optional[str] = None
 
 
 class ProductCreate(ProductBase):
@@ -284,6 +292,14 @@ class ProductUpdate(BaseModel):
     options: Optional[List[ProductOptionCreate]] = None
     ingredients: Optional[List[ProductIngredientCreate]] = None
     preference_sets: Optional[List[PreferenceSetCreate]] = None
+    # Xenia Connect — digital menu overrides
+    digital_visible: Optional[bool] = None
+    digital_available: Optional[bool] = None
+    digital_name: Optional[str] = None
+    digital_description: Optional[str] = None
+    digital_price: Optional[float] = None
+    digital_discount: Optional[float] = None
+    digital_image_url: Optional[str] = None
 
 
 class ProductReorderItem(BaseModel):