docs: document tags filter API and usage

- Add comprehensive API documentation for tags_filter parameter
- Document fuzzy matching behavior with examples
- Add user guide for tag-based filtering
- Document backward compatibility with genre_filter
- Include examples of fuzzy matching ("Sci Fi" → "Science Fiction")

Provides complete documentation for the new tags filter feature,
including API reference and user-facing documentation.

Relates to IMPLEMENTATION_TAGS_FILTER.md Phase 7

docs/developer/api/media-items/search_media_items.md

@@ -24,7 +24,7 @@ Search and filter media items with fuzzy matching support.
 | -------------- | ------- | --------------------------------------------------- |
 | author_filter  | string  | Fuzzy match author field                            |
 | series_filter  | string  | Fuzzy match series field                            |
-| genre_filter   | string  | Fuzzy match genre field                             |
+| tags_filter    | string  | **Fuzzy match tags array**                          |
 | language_filter| string  | Fuzzy match language field                          |
 | year_min       | integer | Minimum copyright year (exact range)                |
 | year_max       | integer | Maximum copyright year (exact range)                |
@@ -35,7 +35,7 @@ Search and filter media items with fuzzy matching support.
 | Parameter  | Type   | Description                                    |
 | ---------- | ------ | ---------------------------------------------- |
 | authors    | string | Search author values for autocomplete dropdown |
-| genres     | string | Search genre values for autocomplete dropdown  |
+| tagss     | string | Search tags values for autocomplete dropdown  |
 | series     | string | Search series values for autocomplete dropdown |
 | languages  | string | Search language values for autocomplete        |
 
@@ -66,7 +66,7 @@ Use double quotes for exact phrase matching:
 
 **Text filters (fuzzy):**
 - `author_filter=asimov` → fuzzy matches author field
-- `genre_filter=scifi` → fuzzy matches genre field
+- `tags_filter=scifi` → fuzzy matches tags field
 
 **Exact filters:**
 - `year_min=2000&year_max=2010` → exact year range
@@ -115,7 +115,7 @@ Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
 Fiction books from 2000-2010:
 
 ```http
-GET /api/media-items/search?library_id=123e4567-e89b-12d3-a456-426614174000&genre_filter=fiction&year_min=2000&year_max=2010
+GET /api/media-items/search?library_id=123e4567-e89b-12d3-a456-426614174000&tags_filter=fiction&year_min=2000&year_max=2010
 Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
 ```
 
@@ -147,8 +147,9 @@ Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
   {
     "id": "uuid",
     "title": "Foundation",
-    "author": "Asimov, Isaac",
-    "library_id": "...",
+    "author": "Isaac Asimov",
+    "tags": ["Science Fiction", "Adventure"],
+    "library_id": "uuid",
     "library_name": "E-Books"
   }
 ]
@@ -172,3 +173,7 @@ Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
 | 400  | Missing library_id           |
 | 401  | Invalid or expired token     |
 | 404  | No results found             |
+
+### Backward Compatibility
+
+The `genre_filter` parameter is **deprecated but still supported** for backward compatibility. It will return 0 results for books imported from Calibre (genre field is NULL).