feat(newznab): support multiple named indexers (#1271)

## Summary

- add a named Newznab indexer table with per-indexer URL and API key
settings
- search every configured indexer and retain the originating indexer
name on each result
- namespace cached release IDs across connections and isolate individual
indexer failures
- preserve the legacy single-indexer settings as a fallback
- support masked API-key cells and trusted SABnzbd prefetching for named
indexers

## Validation

- 121 Newznab and SABnzbd backend tests passed on Python 3.14
- Ruff passed for all changed Python files
- frontend TypeScript and strict lint checks passed
- all 134 frontend unit tests passed
- frontend formatting check passed

## Compatibility

Existing `NEWZNAB_URL` and `NEWZNAB_API_KEY` configurations continue to
work whenever `NEWZNAB_INDEXERS` is empty.

Co-authored-by: Ryan <zab1996@users.noreply.github.com>

src/frontend/src/components/settings/fields/TableField.tsx

@@ -353,12 +353,12 @@ export const TableField = ({ field, value, onChange, disabled }: TableFieldProps
                 );
               }
 
-              // text/path
+              // text/password/path
               return (
                 <div key={col.key} className="flex min-w-0 flex-col gap-1">
                   {mobileLabel}
                   <input
-                    type="text"
+                    type={col.type === 'password' ? 'password' : 'text'}
                     value={toPrimitiveString(cellValue)}
                     onChange={(e) => updateCell(rowIndex, col.key, e.target.value)}
                     placeholder={col.placeholder}

src/frontend/src/types/settings.ts

@@ -146,7 +146,13 @@ export interface TableFieldColumnOption {
   childOf?: string;
 }
 
-export type TableFieldColumnType = 'text' | 'select' | 'multiselect' | 'checkbox' | 'path';
+export type TableFieldColumnType =
+  | 'text'
+  | 'password'
+  | 'select'
+  | 'multiselect'
+  | 'checkbox'
+  | 'path';
 
 export interface TableFieldColumn {
   key: string;