fix: Failed scaling of uploaded signature

[Roshan931]

Fixes #7981

Root cause

Uploaded signature PNG is composited at pixel-size × scale factor instead of being fitted into the stamp box

Changes

• Resize the user signature with Imagick bestfit into the stamp canvas (same aspect-preserving approach already used for the background in prepareBackgroundForPdf) instead of multiplying its own pixel dimensions by the scale factor, plus a regression test asserting the whole signature stays inside the merged canvas.

[welcome]

Thanks for opening your first pull request in this repository! ✌️