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/storage/gradient.py

@@ -0,0 +1,83 @@
+"""Gradient data model.
+
+A Gradient defines a reusable color gradient as a list of color stops.
+Gradients are referenced by ID from effect, gradient, and audio color
+strip sources.  Eight built-in gradients are seeded on first run.
+"""
+
+from dataclasses import dataclass, field
+from datetime import datetime
+from typing import List, Optional
+
+
+@dataclass
+class Gradient:
+    """Persistent gradient definition with color stops."""
+
+    id: str
+    name: str
+    stops: list  # [{"position": float, "color": [R, G, B]}, ...]
+    is_builtin: bool
+    created_at: datetime
+    updated_at: datetime
+    description: Optional[str] = None
+    tags: List[str] = field(default_factory=list)
+
+    def to_dict(self) -> dict:
+        return {
+            "id": self.id,
+            "name": self.name,
+            "stops": self.stops,
+            "is_builtin": self.is_builtin,
+            "description": self.description,
+            "tags": self.tags,
+            "created_at": self.created_at.isoformat(),
+            "updated_at": self.updated_at.isoformat(),
+        }
+
+    @staticmethod
+    def from_dict(data: dict) -> "Gradient":
+        return Gradient(
+            id=data["id"],
+            name=data["name"],
+            stops=data.get("stops", []),
+            is_builtin=data.get("is_builtin", False),
+            description=data.get("description"),
+            tags=data.get("tags", []),
+            created_at=datetime.fromisoformat(data["created_at"]),
+            updated_at=datetime.fromisoformat(data["updated_at"]),
+        )
+
+    @classmethod
+    def create_from_kwargs(
+        cls,
+        *,
+        id: str,
+        name: str,
+        stops: list,
+        is_builtin: bool = False,
+        created_at: datetime,
+        updated_at: datetime,
+        description: Optional[str] = None,
+        tags: Optional[List[str]] = None,
+    ) -> "Gradient":
+        return cls(
+            id=id,
+            name=name,
+            stops=stops,
+            is_builtin=is_builtin,
+            created_at=created_at,
+            updated_at=updated_at,
+            description=description,
+            tags=tags or [],
+        )
+
+    def apply_update(self, **kwargs) -> None:
+        if kwargs.get("name") is not None:
+            self.name = kwargs["name"]
+        if kwargs.get("stops") is not None:
+            self.stops = kwargs["stops"]
+        if kwargs.get("description") is not None:
+            self.description = kwargs["description"]
+        if kwargs.get("tags") is not None:
+            self.tags = kwargs["tags"]