Refactor invoice financials to change iva_factor type from string to numeric

- Updated the `iva_factor` field in the `InvoiceFinancials` model to use a numeric type with precision and scale.
- Adjusted related frontend components and API interfaces to reflect the new numeric type for `iva_factor`.
- Enhanced Alembic migration scripts to accommodate the schema changes, ensuring proper index management and data type conversions.

These changes improve data integrity and consistency for financial calculations in invoices.

backend/api/v1/modules/a76/invoices/models.py

@@ -517,9 +517,9 @@ class InvoiceFinancials(Base, TenantScopedMixin, TimestampMixin):
     iva_mc: Mapped[Optional[float]] = mapped_column(
         Numeric(23, 8), default=0, server_default="0"
     )  # IVAEXPOMC / IVA en MC
-    iva_factor: Mapped[Optional[str]] = mapped_column(
-        String(10)
-    )  # FACTORIVA / Factor IVA (puede ser varchar en imports)
+    iva_factor: Mapped[Optional[float]] = mapped_column(
+        Numeric(23, 8), default=0, server_default="0"
+    )  # FACTORIVA / Factor IVA
     tax_value_me: Mapped[Optional[float]] = mapped_column(
         Numeric(23, 8), default=0, server_default="0"
     )  # VALORIMPUESTOME / Valor impuesto ME