fix: downgrade Logger.error to Logger.warning for known path-not-found failures in MediaImport

[mvanhorn]

Summary

• Downgrade Logger.error to Logger.warning on four call sites in MediaImport that handle known operational path-failure conditions: empty file list from client, empty save_path result, save_path fallback failure, and the :path_not_found/:path_not_accessible branches inside list_files_in_path/1.
• Add operational: true structured metadata key to each warning so operators can filter them in log aggregators and distinguish them from genuine code failures.
• Leave Logger.error in place on actual code defects: failed client info lookup and unhandled import exceptions.

Why this matters

Closes #189. Live instances report ~3,800 :path_not_found telemetry events in a 14-day window. The root cause is the classic self-hosted volume-mount mismatch — the torrent client reports a path like /downloads/complete/<release> that does not exist inside Mydia's container filesystem view. The retry loop (up to 3 attempts) multiplies each underlying failure into several telemetry reports. These are expected operational misconfiguration events, not code defects, and Logger.error is the wrong level for them. Downgrading to Logger.warning keeps the information visible in logs while stopping these known conditions from reaching crash telemetry.

Fixes #189

[github-actions]

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