Make layout blur, opacity, and watermark position apply in rendered videos.

Still-image encodes ignored boxblur and 1s black fills; switch to gblur plus full-length black frames, and honor brand watermark position.

lib/ffmpeg/brand-watermark.ts

@@ -1,20 +1,25 @@
 /**
  * Modular FFmpeg filter fragments for the Songs2VID brand watermark overlay.
- * Always bottom-right with padding; scales to a fraction of frame width.
+ * Honors corner/center position + padding; scales to a fraction of frame width.
  */
+import { overlayXy, type WatermarkPosition } from "../watermark";
+
 export function buildBrandWatermarkOverlayFilters(options: {
   baseLabel: string;
   watermarkInputIndex: number;
   watermarkWidthPx: number;
+  position?: WatermarkPosition;
   offsetX?: number;
   offsetY?: number;
   outLabel?: string;
 }): string[] {
   const ox = options.offsetX ?? 20;
   const oy = options.offsetY ?? 20;
+  const position = options.position ?? "bottom-right";
+  const { x, y } = overlayXy(position, ox, oy);
   const out = options.outLabel ?? "vout";
   return [
     `[${options.watermarkInputIndex}:v]scale=${options.watermarkWidthPx}:-1[wm]`,
-    `[${options.baseLabel}][wm]overlay=W-w-${ox}:H-h-${oy}[${out}]`,
+    `[${options.baseLabel}][wm]overlay=${x}:${y}[${out}]`,
   ];
 }