feat(reviewiq): Add AI synthesis support to dashboard components

Frontend:
- Add Synthesis type with action plan, insights, annotations
- ExecutiveSummary: Accept synthesis prop for AI narrative
- SentimentPie: Accept insight prop for contextual explanation
- IntensityHeatmap: Accept insight + highlightDomain props
- TimelineChart: Accept insight + annotations props
- All components gracefully degrade when synthesis is null

Backend:
- Add Stage 4: Synthesize for generating AI narratives
- Gathers context from classified spans
- Generates executive narrative, section insights, action plan
- Produces timeline annotations and marketing angles
- Stores synthesis in pipeline.executions table

Components show AI insights with purple gradient styling when available,
fall back to existing behavior when synthesis is not yet generated.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

web/components/reviewiq/charts/SentimentPie.tsx

@@ -8,6 +8,8 @@ import { useReviewIQFilters } from '@/contexts/ReviewIQFilterContext';
 
 interface SentimentPieProps {
   data: SentimentDataPoint[];
+  // AI-generated insight (optional - shows when available)
+  insight?: string | null;
 }
 
 // User-friendly sentiment config
@@ -74,7 +76,7 @@ const SENTIMENT_ORDER = ['V+', 'V-', 'V0', 'V±'];
  * User-friendly design with emojis and clear numbers.
  * Click to filter by sentiment.
  */
-export function SentimentPie({ data }: SentimentPieProps) {
+export function SentimentPie({ data, insight }: SentimentPieProps) {
   const { filters, toggleSentiment } = useReviewIQFilters();
 
   // Process data
@@ -185,6 +187,19 @@ export function SentimentPie({ data }: SentimentPieProps) {
         </div>
       </div>
 
+      {/* AI Insight (when available) */}
+      {insight && (
+        <div className="mb-4 p-3 bg-gradient-to-r from-purple-50 to-blue-50 rounded-lg border border-purple-200">
+          <div className="flex items-start gap-2">
+            <span className="text-sm">✨</span>
+            <div>
+              <div className="text-xs font-medium text-purple-600 mb-0.5">AI Insight</div>
+              <p className="text-sm text-gray-700">{insight}</p>
+            </div>
+          </div>
+        </div>
+      )}
+
       {processedData.cards.length === 0 ? (
         <div className="flex items-center justify-center h-64 text-gray-500">
           No sentiment data available