Backend: table groups with prefix/color, auto-numbering, has_active_order flag

TableGroup gains prefix and color columns for display in the PWA zone filter.
Table creation now assigns a global auto-increment number; batch creation uses
group-local label numbering (avoids gaps/conflicts when adding to existing groups).
DELETE table now blocks if an active order exists (soft or hard delete).
Hard delete cascades past orders before removing the table row.
list_tables enriches each TableOut with has_active_order computed server-side.
TableOut no longer requires number in the input payload; TableCreate simplified.

Migration runner refactored to give each ALTER TABLE its own connection so a
no-op (column already exists) doesn't leave a dirty transaction blocking later
migrations. New migrations added for all new columns.

Order.print_logs relationship gains cascade="all, delete-orphan" so print logs
are removed when an order is deleted.

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

local_backend/models/order.py

@@ -21,7 +21,7 @@ class Order(Base):
     closer = relationship("User", foreign_keys=[closed_by], back_populates="orders_closed")
     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")
+    print_logs = relationship("PrintLog", back_populates="order", cascade="all, delete-orphan")
 
 
 class OrderWaiter(Base):