Implement PyTorch fftconvolve backend helper

[FlorianPfaff]

Summary

• Implement pyrecest.backend.signal.fftconvolve for the PyTorch backend using torch.fft.
• Preserve SciPy-compatible mode={full,same,valid} and selected-axis shape semantics.
• Remove PyTorch signal.fftconvolve from unsupported backend capability metadata.
• Add focused PyTorch regression coverage against scipy.signal.fftconvolve.

Validation

• Locally validated the implementation prototype against SciPy for 1-D, 2-D, and selected-axis cases in the execution sandbox.
• Full repository test run was not possible from the sandbox because direct GitHub clone failed with DNS resolution errors; CI should validate the branch in GitHub Actions.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	44.64s
✅ JSON	prettier	7	0	0	0	1.03s
✅ JSON	v8r	7		0	0	4.68s
✅ MARKDOWN	markdownlint	68	0	0	0	1.68s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.55s
✅ PYTHON	black	792	5	0	0	37.57s
✅ PYTHON	isort	792	5	0	0	2.04s
✅ REPOSITORY	checkov	yes		no	no	45.09s
✅ REPOSITORY	gitleaks	yes		no	no	17.17s
✅ REPOSITORY	git_diff	yes		no	no	0.12s
✅ REPOSITORY	secretlint	yes		no	no	22.61s
✅ REPOSITORY	syft	yes		no	no	5.9s
✅ REPOSITORY	trivy-sbom	yes		no	no	6.68s
✅ REPOSITORY	trufflehog	yes		no	no	21.88s
✅ YAML	prettier	11	0	0	0	0.7s
✅ YAML	v8r	11		0	0	10.59s
✅ YAML	yamllint	11		0	0	0.45s

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