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/ReviewIQDashboard.tsx

@@ -112,6 +112,7 @@ function ReviewIQDashboardInner({ jobId, businessId }: ReviewIQDashboardProps) {
         avgRating={data.overview.avg_rating}
         domainScores={data.domain_scores}
         onDomainClick={handleDomainClick}
+        synthesis={data.synthesis}
       />
 
       {/* ═══════════════════════════════════════════════════════════════
@@ -119,8 +120,15 @@ function ReviewIQDashboardInner({ jobId, businessId }: ReviewIQDashboardProps) {
           Side-by-side: How customers feel + What they talk about
           ═══════════════════════════════════════════════════════════════ */}
       <div className="grid grid-cols-1 lg:grid-cols-2 gap-6">
-        <SentimentPie data={data.sentiment.distribution} />
-        <IntensityHeatmap data={data.urt.domains} />
+        <SentimentPie
+          data={data.sentiment.distribution}
+          insight={data.synthesis?.sentiment_insight}
+        />
+        <IntensityHeatmap
+          data={data.urt.domains}
+          insight={data.synthesis?.category_insight}
+          highlightDomain={data.synthesis?.priority_domain}
+        />
       </div>
 
       {/* ═══════════════════════════════════════════════════════════════
@@ -133,7 +141,11 @@ function ReviewIQDashboardInner({ jobId, businessId }: ReviewIQDashboardProps) {
           SECTION 4: TRENDS (Timeline)
           How things change over time
           ═══════════════════════════════════════════════════════════════ */}
-      <TimelineChart data={data.timeline} />
+      <TimelineChart
+        data={data.timeline}
+        insight={data.synthesis?.timeline_insight}
+        annotations={data.synthesis?.timeline_annotations}
+      />
 
       {/* ═══════════════════════════════════════════════════════════════
           SECTION 5: DEEP DIVE (Tables)