feat(catalogues): add uom_group_id field to catalogue reference model

- Add uom_group_id column to CatalogueRef model to store UOM group UUID
- Remove redundant unit field from Inventory model for cleaner data structure
- Maintain uom_group_code field for backward compatibility with existing code
- Enable proper tracking of UOM group associations at the catalogue level

app/catalogues/models/model.py

@@ -30,6 +30,7 @@ class CatalogueRef(Base):
     base_price = Column(Numeric(12, 2))
     track_inventory = Column(Boolean, default=False)
     batch_managed = Column(Boolean, default=False)
+    uom_group_id = Column(Text) # UOM group UUID
     uom_group_code = Column(Text)  # UOM group code (e.g., UOM-VOL-000001)
     status = Column(Text, nullable=False)
     created_at = Column(TIMESTAMP, nullable=False, default=datetime.utcnow)
@@ -171,7 +172,6 @@ class InventoryLevel(BaseModel):
 
 class Inventory(BaseModel):
     """Enhanced inventory configuration model"""
-    unit: str = Field(default="PCS", description="Unit of measurement")
     levels: Dict[str, InventoryLevel] = Field(..., description="Inventory levels by merchant type (ncnf, cnf, distributor, retail)")
     
     # Legacy fields for backward compatibility