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>

static/js/roi_calculator.js

@@ -3,7 +3,7 @@
 //   - 80% du temps de transcription manuelle est économisé
 //   - 220 jours ouvrables/an
 //   - Coût annuel comparé = DictIA 16 = 5 750 $ + (201 $ × 12) = 8 162 $
-function roiCalculator() {
+window.roiCalculator = function roiCalculator() {
   return {
     users: 5,
     hours: 2,
@@ -14,9 +14,8 @@ function roiCalculator() {
     },
     get payback() {
       const annualCost = 5750 + (201 * 12);
-      if (this.savings <= 0) return 999;
-      return Math.max(1, Math.round((annualCost / this.savings) * 12));
+      if (this.savings <= 0) return null;
+      return (annualCost / this.savings) * 12;
     }
   };
-}
-window.roiCalculator = roiCalculator;
+};