Add composable filter templates, skip keepalive for serial devices

Filter Template meta-filter: reference existing PP templates inside others
for composable, DRY filter chains. Filters are recursively expanded at
pipeline build time with cycle detection. New `select` option type with
dynamic choices populated by the API.

Keepalive optimization: serial devices (Adalight, AmbiLED) don't need
keepalive — they hold last frame indefinitely. Check `standby_required`
capability at processor start, skip keepalive sends for serial targets,
and hide keepalive metrics in the UI. Rename "Standby Interval" to
"Keep Alive Interval" throughout the frontend.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/api/schemas/filters.py

@@ -1,6 +1,6 @@
 """Filter-related schemas."""
 
-from typing import Any, Dict, List
+from typing import Any, Dict, List, Optional
 
 from pydantic import BaseModel, Field
 
@@ -17,11 +17,12 @@ class FilterOptionDefSchema(BaseModel):
 
     key: str = Field(description="Option key")
     label: str = Field(description="Display label")
-    type: str = Field(description="Option type (float or int)")
+    type: str = Field(description="Option type (float, int, bool, or select)")
     default: Any = Field(description="Default value")
     min_value: Any = Field(description="Minimum value")
     max_value: Any = Field(description="Maximum value")
     step: Any = Field(description="Step increment")
+    choices: Optional[List[Dict[str, str]]] = Field(default=None, description="Available choices for select type")
 
 
 class FilterTypeResponse(BaseModel):