Fix golangci-lint warnings, update GitHub Actions

[umputun]

• add http.MaxBytesReader (64MB cap) to form handlers (gosec G120)
• replace sort with slices.Sort/SortFunc (revive)
• use min() builtin instead of if/else (modernize)
• extract setupLog helper with secret masking
• add modernize linter to golangci config
• bump docker/setup-qemu-action and setup-buildx-action to v4

[Copilot]

Pull request overview

This PR addresses static-analysis/lint warnings and modernizes parts of the Go codebase and CI configuration, primarily around request body limits, newer stdlib helpers, and GitHub Actions updates.

Changes:

• Add http.MaxBytesReader to form handlers to cap request body size.
• Modernize Go code by switching to slices.Sort/SortFunc and the min() builtin.
• Update lint configuration (enable modernize) and bump Docker build actions in CI.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
backend/rest/server.go	Adds request body size limiting to form-parsing handlers and minor error-handling refactor.
backend/main.go	Refactors logger initialization into a helper intended to include secret masking.
backend/extractor/text.go	Uses min() builtin for snippet sizing logic.
backend/extractor/pics.go	Replaces sort usage with slices/cmp helpers.
backend/.golangci.yml	Enables the modernize linter and normalizes formatter exclusions.
.github/workflows/ci.yml	Bumps Docker QEMU/Buildx setup actions to v4.

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[umputun]

Superseded by recent refactoring — project structure flattened, CI migrated to native ARM64 runners, tests reworked, and all lint fixes applied directly to master.