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/config.py

@@ -41,6 +41,7 @@ class StorageConfig(BaseSettings):
     scene_presets_file: str = "data/scene_presets.json"
     color_strip_processing_templates_file: str = "data/color_strip_processing_templates.json"
     sync_clocks_file: str = "data/sync_clocks.json"
+    gradients_file: str = "data/gradients.json"
 
 
 class MQTTConfig(BaseSettings):