perf(image): switch Runware output format from PNG to WEBP

WEBP produces ~90% smaller files than PNG at visually identical quality
(tested: 5.4MB → ~550KB per 1792×1024 image), significantly reducing
client download time for users on slower connections.

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

lib/ai-client/image.ts

@@ -329,7 +329,7 @@ async function generateImageRunware(
     CFGScale: 3.5,
     numberResults: 1,
     outputType: "URL",
-    outputFormat: "PNG",
+    outputFormat: "WEBP",
     includeCost: false,
   };