fix(rsplat): add prompt and detection_threshold for SAM 3D

SAM 3D requires a segmentation prompt — default "car" fails on
non-car images. Use "everything" with low threshold (0.2) to capture
full scenes including people and backgrounds.

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

server/index.ts

@@ -1035,7 +1035,7 @@ app.post("/api/3d-gen", async (c) => {
 				Authorization: `Key ${FAL_KEY}`,
 				"Content-Type": "application/json",
 			},
-			body: JSON.stringify({ image_url, export_textured_glb: true }),
+			body: JSON.stringify({ image_url, prompt: "everything", detection_threshold: 0.2, export_textured_glb: true }),
 			signal: controller.signal,
 		});
 		clearTimeout(timeout);