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/DashboardSkeleton.tsx

@@ -0,0 +1,74 @@
+'use client';
+
+/**
+ * Loading skeleton for the ReviewIQ Dashboard.
+ */
+export function DashboardSkeleton() {
+  return (
+    <div className="space-y-6 animate-pulse">
+      {/* KPI Cards Skeleton */}
+      <div className="grid grid-cols-2 md:grid-cols-4 gap-4">
+        {Array.from({ length: 8 }).map((_, i) => (
+          <div
+            key={i}
+            className="bg-gray-200 rounded-xl h-28"
+          />
+        ))}
+      </div>
+
+      {/* Charts Grid Skeleton */}
+      <div className="grid md:grid-cols-3 gap-6">
+        <div className="bg-gray-200 rounded-xl h-80" />
+        <div className="bg-gray-200 rounded-xl h-80" />
+        <div className="bg-gray-200 rounded-xl h-80" />
+      </div>
+
+      {/* Timeline Skeleton */}
+      <div className="bg-gray-200 rounded-xl h-96" />
+
+      {/* Tables Skeleton */}
+      <div className="grid md:grid-cols-2 gap-6">
+        <div className="bg-gray-200 rounded-xl h-80" />
+        <div className="bg-gray-200 rounded-xl h-80" />
+      </div>
+    </div>
+  );
+}
+
+/**
+ * Error state component.
+ */
+export function DashboardError({ message, onRetry }: { message: string; onRetry?: () => void }) {
+  return (
+    <div className="flex flex-col items-center justify-center h-96 bg-red-50 rounded-xl border-2 border-red-200">
+      <div className="text-red-600 text-lg font-semibold mb-2">
+        Failed to load dashboard
+      </div>
+      <p className="text-red-500 text-sm mb-4">{message}</p>
+      {onRetry && (
+        <button
+          onClick={onRetry}
+          className="px-4 py-2 bg-red-600 text-white rounded-lg font-semibold hover:bg-red-700 transition-colors"
+        >
+          Try Again
+        </button>
+      )}
+    </div>
+  );
+}
+
+/**
+ * Empty state when no job is selected.
+ */
+export function DashboardEmpty() {
+  return (
+    <div className="flex flex-col items-center justify-center h-96 bg-gray-50 rounded-xl border-2 border-gray-200">
+      <div className="text-gray-600 text-lg font-semibold mb-2">
+        No Job Selected
+      </div>
+      <p className="text-gray-500 text-sm">
+        Select a job to view analytics or run the ReviewIQ pipeline first.
+      </p>
+    </div>
+  );
+}