[ADD] product_pricelist_total_margin: New module for additive margin calculation

- Add use_total_margin field to pricelist items
- Override _compute_price() to sum margins additively instead of compounding
- Support chained pricelists with custom base types (last_purchase_price)
- Add global minimum and maximum margin limits configuration
- Store limits in ir.config_parameter via res.config.settings
- Apply global limits after total margin calculation
- Add comprehensive test suite (9 tests) covering:
  * Basic additive vs compound margin behavior
  * Three-level pricelist chains
  * Global minimum/maximum margin enforcement
  * Rounding and surcharge compatibility
- Add configuration UI in Settings > Sales
- All tests passing (9/9)

This module fixes the issue where chained pricelists were compounding margins
instead of calculating total margins. Example: base 4.68€ with -5% and +25%
now correctly results in 5.616€ (20% total) instead of 5.56€ (compound).

mypy.ini

@@ -3,3 +3,10 @@
 # duplicate module name errors when multiple addons include scripts with the
 # same filename.
 exclude = .*/migrations/.*
+
+# Ignore missing imports from Odoo modules
+[mypy-odoo.*]
+ignore_missing_imports = True
+
+[mypy-odoo]
+ignore_missing_imports = True