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/TimelineChart.tsx

@@ -15,12 +15,16 @@ import {
   ReferenceLine,
 } from 'recharts';
 import { X, TrendingUp, TrendingDown, Minus, Calendar, Filter } from 'lucide-react';
-import type { TimelinePoint, TimeRange } from '../types';
+import type { TimelinePoint, TimeRange, TimelineAnnotation } from '../types';
 import { DOMAIN_LABELS } from '../types';
 import { useReviewIQFilters } from '@/contexts/ReviewIQFilterContext';
 
 interface TimelineChartProps {
   data: TimelinePoint[];
+  // AI-generated insight (optional - shows when available)
+  insight?: string | null;
+  // Timeline annotations from AI (optional - marks key events)
+  annotations?: TimelineAnnotation[] | null;
 }
 
 type ViewMode = 'sentiment' | 'volume' | 'rating';
@@ -45,7 +49,7 @@ const TIME_RANGE_OPTIONS: { value: TimeRange; label: string; description: string
  * User-friendly design with view toggles and interactive brush.
  * Responds to domain/sentiment filters.
  */
-export function TimelineChart({ data }: TimelineChartProps) {
+export function TimelineChart({ data, insight, annotations }: TimelineChartProps) {
   const { filters, setTimeRange, setBrushRange } = useReviewIQFilters();
   const [viewMode, setViewMode] = useState<ViewMode>('sentiment');
   const [localBrushRange, setLocalBrushRange] = useState<{
@@ -264,6 +268,44 @@ export function TimelineChart({ data }: TimelineChartProps) {
         )}
       </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>
+      )}
+
+      {/* Key Events (when annotations available) */}
+      {annotations && annotations.length > 0 && (
+        <div className="mb-4 flex flex-wrap gap-2">
+          {annotations.slice(0, 3).map((annotation, idx) => (
+            <div
+              key={idx}
+              className={`px-3 py-1.5 rounded-full text-xs font-medium flex items-center gap-1 ${
+                annotation.type === 'positive' ? 'bg-green-100 text-green-700' :
+                annotation.type === 'negative' ? 'bg-red-100 text-red-700' :
+                annotation.type === 'event' ? 'bg-blue-100 text-blue-700' :
+                'bg-gray-100 text-gray-700'
+              }`}
+              title={annotation.description}
+            >
+              <span>{
+                annotation.type === 'positive' ? '📈' :
+                annotation.type === 'negative' ? '📉' :
+                annotation.type === 'event' ? '📍' : '•'
+              }</span>
+              <span>{annotation.label}</span>
+            </div>
+          ))}
+        </div>
+      )}
+
       {sortedData.length === 0 ? (
         <div className="flex flex-col items-center justify-center h-80 text-gray-500">
           <Calendar className="w-12 h-12 text-gray-300 mb-2" />