fix(marketing): pricing — honest ROI payback + capped sliders + URL hygiene

- ROI payback now returns raw months; template branches to 'moins d'un mois'
  for sub-month paybacks and 'Payable dès la première année' when savings≤0
  (was rounding up to 'Payback : 1 mois' for ~95% of slider combos)
- Cap sliders: users 1..25 (was 50), hours 0.5..4 (was 8) to keep displayed
  savings in a defensible band (~8.8 M$/yr max instead of 35 M$)
- pricing_card href uses cta_url.rstrip('/') to avoid double-slash if caller
  passes a trailing slash (preempts A-2.8 / B-2.7 regression)
- aria-live polite + aria-atomic on the savings paragraph so screen readers
  announce slider updates
- Cleaner JS module pattern: single window.roiCalculator = function() {...}
- Tests updated for payback ternary; new tests for slider caps, aria-live,
  and double-slash guard

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

templates/macros/pricing_card.html

@@ -35,7 +35,7 @@
       {% endfor %}
     </ul>
     {% from 'macros/button.html' import button %}
-    {{ button('Réserver ' + name, href=cta_url + '/' + slug, variant='primary' if recommended else 'secondary', size='lg') }}
+    {{ button('Réserver ' + name, href=cta_url.rstrip('/') + '/' + slug, variant='primary' if recommended else 'secondary', size='lg') }}
   </div>
 </div>
 {%- endmacro -%}