Add tag-only handoff mode with deferred ARR follow-up

[tomerh2001]

Summary

Implements a configurable tag-only handoff workflow so Decluttarr can defer cleanup ownership to an external service (for example qbit_manage), while optionally triggering ARR follow-up only after external removal is detected.

What Changed

• Added new ARR-removal job controls:
  • action_mode: remove | skip | tag_only
  • handoff_tag
  • deferred_arr_followup
  • followup_trigger (currently on_download_removed)
• Extended JobDefaults and job-level parsing for the above fields.
• Added env parsing support for job-default keys (ACTION_MODE, HANDOFF_TAG, DEFERRED_ARR_FOLLOWUP, FOLLOWUP_TRIGGER).
• Implemented tag_only path in RemovalHandler:
  • Apply handoff tag to matching torrent.
  • If deferred_arr_followup: true, detect whether the download hash is gone from qBittorrent.
  • Trigger ARR queue removal/blocklist follow-up once external removal is detected.
• Added config/docs updates in config/config_example.yaml and README.md.

Tests

• Added new tests for:
  • tag-only handoff waiting behavior
  • deferred follow-up trigger after external removal
  • safe skip when tag handoff is unsupported
  • job defaults + job-level override parsing for new fields
  • env-loading for new job-default keys
• Full suite run locally:
  • python3 -m pytest -q

Related

• Closes Feature: tag-only handoff mode with deferred ARR follow-up (single cleanup owner) #335