Feature: Multi-user request system (#615)

- Adds a comprehensive multi-user request system to the existing
download flow
- Request configuration is policy based. Configure global settings for
content type, or narrow down policy for specific sources (E.g. allow
direct downloads, set prowlarr to request only, block IRC completely,
etc).
- Global policy configuration and per-user overrides for tailored
configs
- Replaced downloads sidebar with ActivitySidebar, combining active
downloads with requests. Admin management of user requests is done here,
and admins have view of downloads from all users. Sidebar can now be
pinned.
- Request either a standard book or a specific release. Release-requests
are used if you permit one source differently than the other. On
book-level requests, admins pick the specific file to be attached to the
fulfilled request.
- Users can request books with a note

This is WIP so some features are still not complete (notifications, more
automatic release selection, among others).

docs/dev/plugin-settings.md

@@ -190,6 +190,31 @@ HeadingField(
 )
 ```
 
+### CustomComponentField
+
+Render a frontend-registered custom settings component while still using the
+decorator-based schema.
+
+```python
+from shelfmark.core.settings_registry import CustomComponentField
+
+CustomComponentField(
+    key="request_policy_editor",
+    component="request_policy_grid",  # frontend registry key
+    label="Request Policy Rules",
+    description="Custom editor for policy defaults and matrix rules.",
+    value_fields=[
+        SelectField(key="REQUEST_POLICY_DEFAULT_EBOOK", label="Default Ebook Mode", default="download"),
+        SelectField(key="REQUEST_POLICY_DEFAULT_AUDIOBOOK", label="Default Audiobook Mode", default="download"),
+        TableField(key="REQUEST_POLICY_RULES", label="Rules", columns=_rule_columns, default=[]),
+    ],
+    wrap_in_field_wrapper=True,  # use standard FieldWrapper label/description layout
+)
+```
+
+When `value_fields` is provided, those backing fields are included in
+serialization/save/validation automatically and are hidden from the default renderer.
+
 ## Common Field Properties
 
 All field types support these common properties:
@@ -206,6 +231,7 @@ All field types support these common properties:
 | `requires_restart` | `bool` | `False` | Whether changes require container restart |
 | `show_when` | `dict` | `None` | Conditional visibility (see below) |
 | `disabled_when` | `dict` | `None` | Conditional disable (see below) |
+| `hidden_in_ui` | `bool` | `False` | Hide from default renderer but keep in schema/save path |
 
 ## Conditional Visibility