docs(api): update GET /api/nfo/{key}/content entry to match the restored endpoint

The API reference for the 'View NFO XML' endpoint was written for the
pre-refactor route shape (/api/nfo/{serie_id}/content with
{serie_id, serie_folder, ...} response fields) that was removed in the
NFO refactor (commits 21af502, a8e5487). The previous fix
(9f52ea0) re-introduced the endpoint as /api/nfo/{key}/content with
{key, folder, ...} response fields, but the docs still described the
old, non-existent shape — anyone reading the API reference and trying
to integrate would have hit a field-name mismatch.

Update the entry in Docs/API.md:
- Path parameter: serie_id -> key (matches the actual route and the
  convention used by the other NFO endpoints in the same file).
- Response fields: serie_id/serie_folder -> key/folder.
- Errors: add 400 (no folder), 500 (file read), 503 (anime_directory
  not configured) — matching what the handler actually raises.
- Source link: point at the new line range in src/server/api/nfo.py.

Scope: only the /content entry. The surrounding /api/nfo/{serie_id}/*
block still describes other legacy endpoints that were removed in the
same refactor — left untouched per 'only the last added endpoint'.

Docs/API.md

@@ -1056,34 +1056,39 @@ Update existing NFO file with fresh TMDB data.
 
 Source: [src/server/api/nfo.py](../src/server/api/nfo.py#L243-L325)
 
-### GET /api/nfo/{serie_id}/content
+### GET /api/nfo/{key}/content
 
-Get NFO file XML content for a series.
+Read the raw `tvshow.nfo` XML for a series. Used by the Anime Settings
+page's "View NFO XML" button to render the on-disk NFO in a `<pre>` block.
 
 **Authentication:** Required
 
 **Path Parameters:**
 
-- `serie_id` (string): Series identifier
+- `key` (string): Series unique key (e.g., `attack-on-titan`)
 
-**Response (200 OK):**
+**Response (200 OK):** [`NfoContentResponse`](../src/server/models/nfo.py)
 
 ```json
 {
-    "serie_id": "one-piece",
-    "serie_folder": "One Piece (1999)",
+    "key": "attack-on-titan",
+    "folder": "Attack on Titan (2013)",
     "content": "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<tvshow>...</tvshow>",
     "file_size": 2048,
-    "last_modified": "2026-01-15T10:30:00"
+    "last_modified": "2026-09-04T17:42:13"
 }
 ```
 
 **Errors:**
 
-- `401 Unauthorized` - Not authenticated
-- `404 Not Found` - Series or NFO not found
+- `400 Bad Request` — Series has no folder configured.
+- `401 Unauthorized` — Not authenticated.
+- `404 Not Found` — Series with the given key does not exist, or its
+  `tvshow.nfo` is missing on disk.
+- `500 Internal Server Error` — Failed to read the NFO file from disk.
+- `503 Service Unavailable` — `settings.anime_directory` is not configured.
 
-Source: [src/server/api/nfo.py](../src/server/api/nfo.py#L328-L397)
+Source: [src/server/api/nfo.py](../src/server/api/nfo.py#L411-L477)
 
 ### GET /api/nfo/{serie_id}/media/status