Expand the svg-to-raster dedicated endpoint test suite from 3 to 10 tests:
- All 7 output formats (png, jpg, webp, avif, tiff, gif, heif)
- Quality setting verification (low vs high quality jpg file sizes)
- DPI setting verification (72 vs 300 dpi png file sizes)
- Preview URL generation for non-browser formats (tiff)
HEIF test skips gracefully if heif-enc is not installed (422 response).
SVG files were rejected by the convert endpoint because
validateImageBuffer only recognized raster magic bytes. This adds
text-based SVG detection, sanitization in the tool factory, and
proper Sharp density handling so SVG-to-raster conversion works
through the standard convert route.