refactor(web): remove client-side BYO API key feature

The BYO (Bring Your Own) API key configuration for LLM and image
generation will be re-implemented via Cloudflare Workers. Remove
the client-side implementation to prepare for that migration.

TTS (text-to-speech) BYO key support is intentionally preserved.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/api/beat-audio/route.ts

@@ -24,7 +24,7 @@ export async function POST(req: Request) {
   }
 
   try {
-    const config = loadEngineConfig(req.headers);
+    const config = loadEngineConfig();
     const result = await requestBeatAudio(config, body);
     if (!result.audio) return new Response(null, { status: 204 });
     const binary = Buffer.from(result.audio.base64, "base64");

app/api/insert-beat/route.ts

@@ -22,7 +22,7 @@ export async function POST(req: Request) {
   }
 
   try {
-    const base = loadEngineConfig(req.headers);
+    const base = loadEngineConfig();
     // See StartRequest.clientTts — BYO clients synth in-browser, so drop server TTS.
     const config = body.clientTts === true ? { ...base, tts: undefined } : base;
     const result = await requestInsertBeat(config, body);

app/api/parse-style-image/route.ts

@@ -51,7 +51,7 @@ export async function POST(req: Request) {
   }
 
   try {
-    const config = loadEngineConfig(req.headers);
+    const config = loadEngineConfig();
     const raw = await analyzeImageDataUrl(
       config.vision,
       body.imageDataUrl,

app/api/scene/route.ts

@@ -32,7 +32,7 @@ export async function POST(req: Request) {
   }
 
   try {
-    const base = loadEngineConfig(req.headers);
+    const base = loadEngineConfig();
     // See StartRequest.clientTts — BYO clients synth in-browser, so drop server TTS.
     const config = body.clientTts === true ? { ...base, tts: undefined } : base;
     const result = await requestScene(config, body);

app/api/start/route.ts

@@ -41,7 +41,7 @@ export async function POST(req: Request) {
   }
 
   try {
-    const base = loadEngineConfig(req.headers);
+    const base = loadEngineConfig();
     // BYO key: the browser provisions + synths voices directly against Xiaomi
     // (key never reaches us), so strip server-side TTS so the engine skips all
     // provisioning + synth. See StartRequest.clientTts.

app/api/vision/route.ts

@@ -42,7 +42,7 @@ export async function POST(req: Request) {
   }
 
   try {
-    const config = loadEngineConfig(req.headers);
+    const config = loadEngineConfig();
     const result = await visionDecide(config, body);
     return NextResponse.json(result);
   } catch (err) {