fix: improve error handling, url redaction and documentation

[ymettier]

Summary

• fix: cancel context before exit and add signal handling (gosec linter)
• fix: include HTTP status code and text in fetch error
• fix: remove stale .part file on backup write failure
• fix: redact credentials from logged URLs (new logger.RedactURL helper)
• docs: document HTTP client per-request design
• docs: document arm (32-bit) in supported build platforms
• docs: clarify backup_nb counts every retrieval round including retries
• test: add coverage for logger.RedactURL
• test: cover testutil error paths, reach 100% coverage
• test: cover backupFile body-close error path

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
main.go	80.95%	4 Missing ⚠️

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