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/__init__.py

@@ -23,6 +23,7 @@ from .routes.scene_presets import router as scene_presets_router
 from .routes.webhooks import router as webhooks_router
 from .routes.sync_clocks import router as sync_clocks_router
 from .routes.color_strip_processing import router as cspt_router
+from .routes.gradients import router as gradients_router
 
 router = APIRouter()
 router.include_router(system_router)
@@ -46,5 +47,6 @@ router.include_router(scene_presets_router)
 router.include_router(webhooks_router)
 router.include_router(sync_clocks_router)
 router.include_router(cspt_router)
+router.include_router(gradients_router)
 
 __all__ = ["router"]