fix(exec-dashboard): replace time-dependent throughput with Pipeline Conversion %

'Validation Throughput (tests/week)' was time-dependent — director wanted
an activity-based metric instead.

New metric: Pipeline Conversion Rate
  formula: validated / (validated + rejected + in_review) × 100
  unit: %  (no time reference)
  meaning: 'of all tests that have entered validation, X% succeeded'
  trend: declining if in_review backlog > validated count,
         improving if conversion ≥ 80%, stable otherwise

Backend: calculate_validation_throughput() rewritten — same API key
(tests_per_week) kept for compatibility, new conversion_rate field added.
Frontend: label → 'Pipeline Conversion', unit → '%', tooltip updated.

frontend/src/pages/ExecutiveDashboardPage.tsx

@@ -498,11 +498,11 @@ export default function ExecutiveDashboardPage() {
           tooltip={{ description: "Percentage of executed attacks that Blue Team successfully detected. 100% means every simulated attack triggered an alert.", context: "Higher is better. Below 60% indicates significant detection gaps." }}
         />
         <KPICard
-          label="Validation Throughput"
-          value={opMetrics?.validation_throughput?.tests_per_week ?? 0}
-          unit="/week"
+          label="Pipeline Conversion"
+          value={opMetrics?.validation_throughput?.conversion_rate ?? opMetrics?.validation_throughput?.tests_per_week ?? 0}
+          unit="%"
           trend={opMetrics?.validation_throughput?.trend}
-          tooltip={{ description: "Number of tests fully validated (approved by Red + Blue leads) per week. Shows how productive the security testing programme is.", context: "Increasing trend = programme is accelerating. Decreasing may indicate process bottlenecks." }}
+          tooltip={{ description: "Percentage of tests that have entered the validation phase and been successfully approved. Formula: Validated ÷ (Validated + Rejected + In Review) × 100.", context: "100% = all reviewed tests approved. < 60% = quality or process issues. High backlog (many In Review) lowers this score." }}
         />
       </div>