Enlarge watermark badge for preview/encode parity and document video title fields.

Fix double-scaled badge preview, bump default watermark to 42% frame width, and update OSS docs/API for title vs songTitle separation and shared typography.

lib/ffmpeg/encode.ts

@@ -13,6 +13,10 @@ import {
   buildArtTrackFilterComplex,
   type LayoutSettings,
 } from "../layout";
+import {
+  WATERMARK_WIDTH_FRACTION,
+  watermarkFontSizeForWidth,
+} from "../preview-typography";
 import { getWatermarkPath } from "../storage";
 import {
   buildDrawtextFilter,
@@ -113,8 +117,8 @@ export async function encodeVideo(options: {
 
   const settings = normalizeWatermarkSettings(options.watermark, options.includeWatermark);
   const layout = options.layout?.template ? options.layout : null;
-  const watermarkWidth = Math.max(1, Math.round(res.width * 0.32));
-  const fontSize = Math.max(16, Math.round(res.width * 0.018));
+  const watermarkWidth = Math.max(1, Math.round(res.width * WATERMARK_WIDTH_FRACTION));
+  const fontSize = watermarkFontSizeForWidth(res.width);
   const fontfile = await resolveFontfileEscaped(settings);
 
   const args = ["-y", "-loop", "1", "-r", "1", "-i", options.imagePath, "-i", options.audioPath];