scaleImageFromBase64 still decodes the full raster #309

Symptom

ImageTransformer.scaleImageFromURL reads with readSubsampled (ImageReadParam.setSourceSubsampling) before scaling. scaleImageFromBase64 still does ImageIO.read on the full decoded bytes, then scales.

The URL path landed in a9349b8c (fix(backend): subsample URL images before scale) so a remote bitmap is not loaded into the heap at full size. The data-URI path was not changed. A large base64 image still allocates the full BufferedImage.

Same code on origin/dev and on 374e1bbe. SVG data URIs already skip raster scaling.

Why this is not on the LTR pointer PR

The URL subsample shipped with the Boot 4.1.1 bump. It is not part of sitmun/sitmun-application-stack#97 (backend 374e1bbe), and that commit does not change scaleImageFromBase64.

Acceptance

• scaleImageFromBase64 uses the same source subsampling as scaleImageFromURL before scaleImage.
• ImageTransformerTest.pngDataUriIsStillScaled still passes: a PNG data URI is stored as a scaled PNG of the configured size.
• SVG data URIs stay stored as image/svg+xml without rasterizing.