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/models/product.py

@@ -36,6 +36,15 @@ class Product(Base):
     image_url = Column(String, nullable=True)
     sort_order = Column(Integer, default=0, nullable=False)
 
+    # Xenia Connect — digital menu overrides
+    digital_visible = Column(Integer, default=1, nullable=False)
+    digital_available = Column(Integer, default=1, nullable=False)
+    digital_name = Column(String, nullable=True)
+    digital_description = Column(String, nullable=True)
+    digital_price = Column(Float, nullable=True)
+    digital_discount = Column(Float, default=0.0, nullable=False)
+    digital_image_url = Column(String, nullable=True)
+
     category = relationship("Category", back_populates="products")
     printer_zone = relationship("Printer", back_populates="products")
     quick_options = relationship("ProductQuickOption", back_populates="product", cascade="all, delete-orphan")