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/dependencies.py

@@ -21,6 +21,7 @@ from wled_controller.storage.automation_store import AutomationStore
 from wled_controller.storage.scene_preset_store import ScenePresetStore
 from wled_controller.storage.sync_clock_store import SyncClockStore
 from wled_controller.storage.color_strip_processing_template_store import ColorStripProcessingTemplateStore
+from wled_controller.storage.gradient_store import GradientStore
 from wled_controller.core.automations.automation_engine import AutomationEngine
 from wled_controller.core.backup.auto_backup import AutoBackupEngine
 from wled_controller.core.processing.sync_clock_manager import SyncClockManager
@@ -114,6 +115,10 @@ def get_cspt_store() -> ColorStripProcessingTemplateStore:
     return _get("cspt_store", "Color strip processing template store")
 
 
+def get_gradient_store() -> GradientStore:
+    return _get("gradient_store", "Gradient store")
+
+
 # ── Event helper ────────────────────────────────────────────────────────
 
 
@@ -157,6 +162,7 @@ def init_dependencies(
     sync_clock_store: SyncClockStore | None = None,
     sync_clock_manager: SyncClockManager | None = None,
     cspt_store: ColorStripProcessingTemplateStore | None = None,
+    gradient_store: GradientStore | None = None,
 ):
     """Initialize global dependencies."""
     _deps.update({
@@ -178,4 +184,5 @@ def init_dependencies(
         "sync_clock_store": sync_clock_store,
         "sync_clock_manager": sync_clock_manager,
         "cspt_store": cspt_store,
+        "gradient_store": gradient_store,
     })