Allow splitting config, downloads and media paths via .env

Adds optional CONFIG_DIR, DOWNLOADS_DIR and MEDIA_DIR env variables so
users can spread configs, downloads and media across different drives
without restructuring ROOT_DIR. Each falls back to its ROOT_DIR-based
default when left empty, so existing setups keep working unchanged.

The -arr services (sonarr, radarr, lidarr, mylar3) now use a single
${DOWNLOADS_DIR}:/data parent bind with a ${MEDIA_DIR}:/data/media
overlay. This preserves TRaSH's single-/data-mount semantics inside
the container (hardlinks still work on one filesystem) while letting
the host-side media tree live wherever the user wants.

README documents the new variables, the generated folder structure,
recommended storage layouts, and the same-filesystem requirement for
hardlinks between downloads and media.

main.py

@@ -33,6 +33,16 @@ def take_directory_input():
             return ans
         print('Please make sure the path is absolute, meaning it starts at the root of your filesystem and starts with "/":', end=' ')
 
+def take_optional_directory_input(default):
+    """Reads a directory override, or returns ``default`` on an empty response."""
+    while True:
+        ans = input()
+        if ans == '':
+            return default
+        if ans[0] == '/':
+            return ans[:-1] if ans[-1] == '/' else ans
+        print(f'Please provide an absolute path (starting with "/") or press enter to use "{default}":', end=' ')
+
 def get_system_timezone():
     tz_path = '/etc/localtime'
 
@@ -121,13 +131,35 @@ def main():
     print('Where would you like to keep your files?', end=' ')
     root_dir = take_directory_input()
 
+    default_config_dir = f'{root_dir}/config'
+    default_downloads_dir = f'{root_dir}/data'
+    default_media_dir = f'{root_dir}/data/media'
+
+    print('\nBy default all configs, downloads and media go under the root directory you just picked.')
+    print('You can override any of these three with an absolute path, or press enter to accept the default.')
+    print('NOTE: hardlinks/atomic moves only work when downloads and media are on the same filesystem.')
+
+    print(f'Config directory [{default_config_dir}]:', end=' ')
+    config_dir = take_optional_directory_input(default_config_dir)
+    print(f'Downloads directory [{default_downloads_dir}]:', end=' ')
+    downloads_dir = take_optional_directory_input(default_downloads_dir)
+    print(f'Media directory [{default_media_dir}]:', end=' ')
+    media_dir = take_optional_directory_input(default_media_dir)
+
     compose = open('docker-compose.yml', 'w')
     compose.write(
         '---\n'
         'services:\n'
     )
 
-    container_config = ContainerConfig(root_dir, timezone, plex_claim=plex_claim)
+    container_config = ContainerConfig(
+        root_dir,
+        timezone,
+        plex_claim=plex_claim,
+        config_dir=config_dir,
+        downloads_dir=downloads_dir,
+        media_dir=media_dir,
+    )
 
     for service in services:
         compose.write(getattr(container_config, service)())
@@ -137,7 +169,12 @@ def main():
     print("Do you want to also generate the required folder structure and permissions? (this is required for first time setup) [Y/n]: ")
     generate_permissions = take_boolean_input()
     if generate_permissions:
-        permission_setup = UserGroupSetup(root_dir=root_dir)
+        permission_setup = UserGroupSetup(
+            root_dir=root_dir,
+            config_dir=config_dir,
+            downloads_dir=downloads_dir,
+            media_dir=media_dir,
+        )
         for service in services:
             try:
                 getattr(permission_setup, service)()