fix: encode task pagination filters

[saurabhhhcodes]

Summary

• Replace manual task pagination query-string concatenation with urllib.parse.urlencode.
• Preserve existing pagination response fields and filter behavior.
• Add a regression test for filtered next links containing spaces, slashes, and ampersands.

Closes #466

Tests

• PYTHONPYCACHEPREFIX=/private/tmp/secuscan-pycache python3 -m py_compile backend/secuscan/routes.py testing/backend/test_task_pagination.py
• git diff --check
• PYTHONPATH=/private/tmp/secuscan-stubs: /private/tmp/secuscan-466/venv311/bin/python -m pytest testing/backend/test_task_pagination.py -q

Note: the full backend requirements install was blocked locally by pycairo needing system cairo/pkg-config through the PDF reporting dependency; the focused pagination suite was run with a temporary local xhtml2pdf import stub because this route test does not exercise PDF generation.

[saurabhhhcodes]

CI is green on this PR now. Current passing checks: artifact-check, detect-changes, formatting-hygiene, backend-lint, frontend-checks, backend-tests, and benchmark.\n\nThe change is scoped to #466: URL-encoding task pagination links when filters are present. Could you please review when available, and apply/inherit the issue scoring labels on the PR if this is accepted for GSSoC scoring?

[utksh1]

Looks good. Pagination links now use structured URL encoding and include focused coverage for encoded filter values. CI is passing.