feat: add keep_downloads option and prune completed TODO items

[bugrax]

Summary

Works through the README TODO list — implements the one actionable item and prunes the rest.

keep_downloads option (new)

Adds a keep_downloads config flag (default false, so existing behaviour is unchanged). When true, the locally downloaded files are kept in download_directory after the *arr imports them, instead of being deleted. Useful when the *arr copies imports (rather than hardlink/move) and you want to keep the original. Only affects local files — put.io transfers/files are still cleaned up as usual.

Wired through the config struct, the Figment defaults, and the generated config template, and gated in watch_for_import.

TODO cleanup

• "Better error handling and retry behavior" — done across the download resume/retry loop, request/stream timeouts, and worker-resilience work (Logs get bloated if sonarr or Radarr are not properly configured, and eventually fill docker host disk #21, watch_for_import loops forever for transfers that never fully import (e.g. samples), eventually stalling all downloads #30, Downloads can hang forever in reqwest send() (no request timeout), freezing all download workers #32, Orchestration deadlocks/parks with a large backlog: workers go idle after 'generating targets', nothing downloads #34).
• "The session ID provided is hard coded. Not sure if it matters." — it doesn't; the *arr clients work fine against the fixed session id.
• "Add option to not delete downloads" — implemented here.

Left the two genuinely open items (better transfer→import mapping for samples; auto-selecting the fastest put.io proxy).

Testing

Builds cleanly; default (keep_downloads = false) keeps the current delete-after-import behaviour.

[Copilot]

Pull request overview

Adds a keep_downloads configuration option so users can preserve locally downloaded files after *arr import (useful for copy-based imports), and updates docs/templates accordingly. Also removes completed items from the README TODO list.

Changes:

• Add keep_downloads to configuration (serde + Figment defaults) and document it in templates/README.
• Gate local post-import deletion in watch_for_import on keep_downloads.
• Prune completed TODO bullets from README.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/utils.rs	Updates the generated config template to include keep_downloads.
src/main.rs	Adds keep_downloads to the config struct and Figment defaults.
src/download_system/orchestration.rs	Makes local cleanup after import conditional on keep_downloads.
README.md	Documents keep_downloads and prunes completed TODO items.

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[bugrax]

Both addressed in 934cb1f:

1. Blocking delete + panic — the post-import cleanup now uses async tokio::fs and logs a failure (warn!) instead of unwrap()/panic!, so a failed delete (permissions, a concurrent removal, the path already gone) can't take the process down or stall the runtime.
2. Doc accuracy — the keep_downloads note now says *arr instead of listing sonarr/radarr/whisparr, since lidarr and [arrs.*] instances are supported too.