feat: add gradient entity with CRUD API and storage

Reusable gradient definitions with built-in presets (rainbow, sunset,
ocean, etc.) and user-created gradients. Includes model, JSON store,
Pydantic schemas, REST routes (list/create/update/clone/delete), and
backup/restore integration.

server/src/wled_controller/api/routes/backup.py

@@ -54,6 +54,7 @@ STORE_MAP = {
     "color_strip_processing_templates": "color_strip_processing_templates_file",
     "automations": "automations_file",
     "scene_presets": "scene_presets_file",
+    "gradients": "gradients_file",
 }
 
 _SERVER_DIR = Path(__file__).resolve().parents[4]