feat(confidence-engine): add investigation overview synthesis seam

app/api/cases/overview/route.js

@@ -0,0 +1,68 @@
+/**
+ * Investigation Overview synthesis API route.
+ *
+ * Route: POST /api/cases/overview
+ *
+ * Thin route pattern — no overview business logic here.
+ */
+
+import { getProvider } from "@/lib/llm/provider.js";
+import { synthesizeInvestigationOverview } from "@/lib/graph/investigation-overview-synthesis.js";
+
+export async function POST(request) {
+  try {
+    const body = await request.json();
+
+    if (!body || typeof body !== "object") {
+      return Response.json(
+        { success: false, stage: "request_validation", error: "Invalid request body" },
+        { status: 400 }
+      );
+    }
+
+    const { situationGraph, findings, plausibleInterpretations } = body;
+
+    if (!situationGraph) {
+      return Response.json(
+        { success: false, stage: "request_validation", error: "Missing situationGraph" },
+        { status: 400 }
+      );
+    }
+
+    const result = await synthesizeInvestigationOverview(
+      { situationGraph, findings, plausibleInterpretations },
+      {
+        provider: getProvider(),
+        modelName: process.env.OLLAMA_MODEL ?? null,
+      }
+    );
+
+    return Response.json(
+      { success: true, understanding: result.understanding, plausibleInterpretations: result.plausibleInterpretations },
+      { status: 200 }
+    );
+  } catch (error) {
+    if (error instanceof SyntaxError) {
+      return Response.json(
+        { success: false, stage: "request_validation", error: "Invalid JSON request body" },
+        { status: 400 }
+      );
+    }
+
+    if (error.statusCode) {
+      return Response.json(
+        {
+          success: false,
+          stage: error.statusCode === 400 ? "request_validation" : "provider",
+          error: error.message ?? "Overview synthesis failed",
+        },
+        { status: error.statusCode }
+      );
+    }
+
+    return Response.json(
+      { success: false, stage: "internal", error: "Internal server error" },
+      { status: 500 }
+    );
+  }
+}