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/schemas/gradients.py

@@ -0,0 +1,51 @@
+"""Gradient schemas (CRUD)."""
+
+from datetime import datetime
+from typing import List, Optional
+
+from pydantic import BaseModel, Field
+
+
+class GradientStopSchema(BaseModel):
+    """A single gradient color stop."""
+
+    position: float = Field(description="Position along gradient (0.0-1.0)", ge=0.0, le=1.0)
+    color: List[int] = Field(description="RGB color [R, G, B]", min_length=3, max_length=3)
+
+
+class GradientCreate(BaseModel):
+    """Request to create a gradient."""
+
+    name: str = Field(description="Gradient name", min_length=1, max_length=100)
+    stops: List[GradientStopSchema] = Field(description="Color stops", min_length=2)
+    description: Optional[str] = Field(None, description="Optional description", max_length=500)
+    tags: List[str] = Field(default_factory=list, description="User-defined tags")
+
+
+class GradientUpdate(BaseModel):
+    """Request to update a gradient."""
+
+    name: Optional[str] = Field(None, description="Gradient name", min_length=1, max_length=100)
+    stops: Optional[List[GradientStopSchema]] = Field(None, description="Color stops", min_length=2)
+    description: Optional[str] = Field(None, description="Optional description", max_length=500)
+    tags: Optional[List[str]] = None
+
+
+class GradientResponse(BaseModel):
+    """Gradient response."""
+
+    id: str = Field(description="Gradient ID")
+    name: str = Field(description="Gradient name")
+    stops: List[GradientStopSchema] = Field(description="Color stops")
+    is_builtin: bool = Field(description="Whether this is a built-in gradient")
+    description: Optional[str] = Field(None, description="Description")
+    tags: List[str] = Field(default_factory=list, description="User-defined tags")
+    created_at: datetime = Field(description="Creation timestamp")
+    updated_at: datetime = Field(description="Last update timestamp")
+
+
+class GradientListResponse(BaseModel):
+    """List of gradients."""
+
+    gradients: List[GradientResponse] = Field(description="List of gradients")
+    count: int = Field(description="Number of gradients")