Add torrent post-import category action (#1148)

## Summary

- add a **Change Category** torrent completion action and conditionally
show its post-import category/label setting
- update kept torrents only after a successful library import
- support qBittorrent categories, Transmission labels, Deluge's Label
plugin, and rTorrent's `custom1` label
- preserve existing Keep/Remove behavior and document the new
environment setting

## Behavior

Category changes happen from `post_process_cleanup`, after output
transfer and post-processing complete. This keeps the existing hardlink
flow unchanged. An empty post-import category is a no-op, and client API
failures are logged without turning a successful library import into a
failure.

## Validation

- `pytest -n 0 tests/prowlarr/test_qbittorrent_client.py
tests/prowlarr/test_transmission_client.py
tests/prowlarr/test_deluge_client.py
tests/prowlarr/test_rtorrent_client.py tests/prowlarr/test_handler.py
tests/config/test_generate_env_docs.py` — 161 passed
- `ruff check` on all changed Python files — passed
- `basedpyright` on the changed client implementation files — passed
- full `basedpyright shelfmark/download/clients` currently reports two
pre-existing errors in the new debrid connection-test code at
`settings.py:546` and `settings.py:563`, outside this PR's diff
- multi-architecture Docker images built successfully for `linux/amd64`
and `linux/arm64`

docs/environment-variables.md

@@ -1534,7 +1534,8 @@ How long to keep cached search results before they expire.
 | `RTORRENT_LABEL` | Label to assign to ebook downloads in rTorrent | string | `cwabd` |
 | `RTORRENT_AUDIOBOOK_LABEL` | Label to assign to audiobook downloads in rTorrent (falls back to Book Label if not set) | string | _none_ |
 | `RTORRENT_DOWNLOAD_DIR` | Server-side directory where torrents are downloaded (optional, uses rTorrent default if not specified) | string | _none_ |
-| `PROWLARR_TORRENT_ACTION` | Remove deletes the torrent from your client immediately after import (stops seeding, files are kept); Keep leaves it in the client to continue seeding | string (choice) | `keep` |
+| `PROWLARR_TORRENT_ACTION` | Choose whether to keep, remove, or move the torrent to another category or label after import | string (choice) | `keep` |
+| `PROWLARR_TORRENT_POST_IMPORT_CATEGORY` | Category or label to assign after a successful import | string | _empty string_ |
 | `PROWLARR_USENET_CLIENT` | Choose which usenet client to use | string (choice) | _empty string_ |
 | `NZBGET_URL` | URL of your NZBGet instance | string | _none_ |
 | `NZBGET_USERNAME` | NZBGet control username | string | `nzbget` |
@@ -1798,11 +1799,20 @@ Server-side directory where torrents are downloaded (optional, uses rTorrent def
 
 **Torrent Completion Action**
 
-Remove deletes the torrent from your client immediately after import (stops seeding, files are kept); Keep leaves it in the client to continue seeding
+Choose whether to keep, remove, or move the torrent to another category or label after import
 
 - **Type:** string (choice)
 - **Default:** `keep`
-- **Options:** `keep` (Keep), `remove` (Remove)
+- **Options:** `keep` (Keep), `remove` (Remove), `change_category` (Change Category)
+
+#### `PROWLARR_TORRENT_POST_IMPORT_CATEGORY`
+
+**Post-Import Category**
+
+Category or label to assign after a successful import
+
+- **Type:** string
+- **Default:** _empty string_
 
 #### `PROWLARR_USENET_CLIENT`