perf(insights): send snapshots as compact JSON and size the cap against it

The status cards, the briefing, the derived assessments and the Coach
snapshot drop the two-space indentation from the JSON they hand the model.
The content is unchanged (the hash gate hashes the object, so stored hashes
stay valid). MAX_SNAPSHOT_CHARS is now measured on the compact string: 14 000
for the status cards and the Coach, which every snapshot that fitted the old
24 000 pretty-printed characters still fits, so the model reads the same data
for fewer tokens.

Refs #1217

src/app/api/insights/generate/route.ts

@@ -671,7 +671,9 @@ export const POST = apiHandler((request: NextRequest) =>
     const compactFeatures = compactSections(
       features as unknown as Record<string, unknown>,
     );
-    const featuresJson = JSON.stringify(compactFeatures, null, 2);
+    // Compact on the wire: indentation is about 40% of a pretty-printed
+    // payload, and the model reads the structure without it (#1217).
+    const featuresJson = JSON.stringify(compactFeatures);
     if (payloadDowngraded) {
       annotate({
         meta: {