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.

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[vitormattos]

Would be best to use the constant SCALE_FACTOR_MIN because duplicating the value here couples the test to that implementation detail. Ideally the test should exercise the behavior without needing to know the exact minimum value.

[vitormattos]

Maybe it would be good to use #[DataProvider('signatureDimensionsProvider')] here to cover different aspect ratios:

public static function signatureDimensionsProvider(): array {
	return [
		'same aspect ratio' => [1400, 400],
		'wider signature' => [1400, 200],
		'taller signature' => [700, 800],
	];
}

This would keep the regression case and also verify the new behavior with wider and taller signatures.