feat(web,engine): custom style — image upload, AI-extract prompt, painter ref

自定义画风入口里加上传按钮：客户端把图缩到 512px webp(base64)，传到新
路由 /api/parse-style-image，vision LLM 解析成英文 style prompt 回填 textarea；
图本身随 sessionStorage → /api/start → Session.styleReferenceImage 透传，
painter.collectReferenceImages 把它置于 slot 0，整局每一幕都作为 reference
图锚定画风（brush / color / mood），比 priorScene 优先级更高。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/api/start/route.ts

@@ -6,6 +6,11 @@ import { loadEngineConfig } from "@/lib/config";
 export const runtime = "nodejs";
 export const maxDuration = 60;
 
+// Matches /api/vision and /api/parse-style-image — the user's resized 512px
+// webp is ~30-80 KB; this caps pathological direct-API payloads (which would
+// then ride along in every subsequent /api/scene request body via session).
+const MAX_STYLE_REF_BYTES = 3 * 1024 * 1024;
+
 export async function POST(req: Request) {
   let body: StartRequest;
   try {
@@ -20,6 +25,20 @@ export async function POST(req: Request) {
       { status: 400 },
     );
   }
+  if (typeof body.styleReferenceImage === "string") {
+    if (!body.styleReferenceImage.startsWith("data:image/")) {
+      return NextResponse.json(
+        { error: "styleReferenceImage must be a data:image/... base64 URL" },
+        { status: 400 },
+      );
+    }
+    if (body.styleReferenceImage.length > MAX_STYLE_REF_BYTES) {
+      return NextResponse.json(
+        { error: `styleReferenceImage exceeds ${MAX_STYLE_REF_BYTES} bytes` },
+        { status: 413 },
+      );
+    }
+  }
 
   try {
     const config = loadEngineConfig();