feat: add download management actions

[roziscoding]

Summary

• add Management API endpoints to cancel, retry, and delete downloads by numeric record ID
• persist the last failed operation so transfer retries resume .part files and import retries skip re-downloading
• coordinate transfer/import/delete operations to prevent same-infohash sibling and filesystem races
• add responsive UI actions with confirmations, pending state, notifications, and refresh
• avoid Sonarr manual-import discovery failures when a new series folder does not exist yet

API

• POST /downloads/:id/cancel
• POST /downloads/:id/retry
• DELETE /downloads/:id

Verification

• bun test apps packages — 447 passed
• focused concurrency re-review — 104 passed
• mise run lint
• mise run typecheck
• git diff --check

Notes

The retry endpoint dispatches from persisted operation state rather than display status. Failed manual imports are retried directly through Radarr/Sonarr without downloading the file again.

Greptile Summary

Adds download-management operations across the backend and management UI.

• Adds cancel, retry, and delete endpoints keyed by numeric download ID.
• Persists failed-operation state to distinguish transfer retries from import retries.
• Coordinates transfer, import, deletion, and artifact-path ownership.
• Adds responsive UI actions, confirmations, notifications, and pending states.
• Adjusts Sonarr manual-import discovery for missing series folders.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains; the shared deletion-path reservation prevents a retry from starting once deletion begins, while JavaScript run-to-completion ensures deletion synchronously obtains and aborts an already-started transfer before awaiting its completion.

_{Reviews (5): Last reviewed commit: "fix(sonarr): avoid missing series folder..." | Re-trigger Greptile}