Complete task 4: Document series filter feature

- Updated instructions.md to mark task 4 as complete
- Added filter documentation to API.md with examples
- All TODO items now completed

docs/API.md

@@ -203,7 +203,7 @@ List library series that have missing episodes.
 | `page` | int | 1 | Page number (must be positive) |
 | `per_page` | int | 20 | Items per page (max 1000) |
 | `sort_by` | string | null | Sort field: `title`, `id`, `name`, `missing_episodes` |
-| `filter` | string | null | Filter term |
+| `filter` | string | null | Filter: `no_episodes` (shows only series with no downloaded episodes in folder) |
 
 **Response (200 OK):**
 
@@ -220,6 +220,11 @@ List library series that have missing episodes.
 ]
 ```
 
+**Example with filter:**
+```bash
+GET /api/anime?filter=no_episodes
+```
+
 Source: [src/server/api/anime.py](../src/server/api/anime.py#L155-L303)
 
 ### GET /api/anime/search

docs/instructions.md

@@ -125,7 +125,7 @@ For each task completed:
 
 ✅ **COMPLETED (3):** Added database transactions - All database operations properly use session context managers with automatic commit/rollback
 
-**TODO (4):** Create series filter to filter all series with no episodes found in folder
+✅ **COMPLETED (4):** Created series filter to filter all series with no episodes found in folder - Added `filter=no_episodes` parameter to list_anime endpoint and get_series_with_no_episodes() database method
 
 ---