feat: printer duplicate copies setting (Feature 1)

- Add `duplicates` column (0-9) to printers table via migration
- Print loop repeats job 1+duplicates times per printer zone
- PrinterForm in Settings > Print now has ΑΝΤΙΓΡΑΦΑ (0-9) field
- PrinterRow shows amber badge when duplicates > 0

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

local_backend/models/printer.py

@@ -13,6 +13,7 @@ class Printer(Base):
     is_active = Column(Boolean, default=True, nullable=False)
     protocol = Column(String, default="escpos_tcp", nullable=False)
     line_width = Column(Integer, default=48, nullable=False)
+    duplicates = Column(Integer, default=0, nullable=False)
 
     products = relationship("Product", back_populates="printer_zone")
     print_logs = relationship("PrintLog", back_populates="printer")