feat(import): add fast-fail segment checks

[javi11]

Summary

• Add import.fast_fail_segment_checks to stat a configurable random sample of eligible media/archive NZB segments before import processing.
• Fail NZB-backed imports early when a selected segment is unreachable, while leaving the check disabled by default.
• Expose the setting through the config API, sample config, and frontend import settings.

Test Plan

• go test ./internal/importer/validation -count=1
• go test ./internal/config -count=1
• go test ./internal/api -count=1
• go test ./internal/importer/... -count=1
• bunx tsc --noEmit