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/routes/templates.py

@@ -12,6 +12,7 @@ from wled_controller.api.auth import AuthRequired
 from wled_controller.api.dependencies import (
     get_device_store,
     get_picture_source_store,
+    get_pp_template_store,
     get_processor_manager,
     get_template_store,
 )
@@ -385,26 +386,44 @@ async def test_template(
 # ===== FILTER TYPE ENDPOINTS =====
 
 @router.get("/api/v1/filters", response_model=FilterTypeListResponse, tags=["Filters"])
-async def list_filter_types(_auth: AuthRequired):
+async def list_filter_types(
+    _auth: AuthRequired,
+    pp_store=Depends(get_pp_template_store),
+):
     """List all available postprocessing filter types and their options schemas."""
     all_filters = FilterRegistry.get_all()
+
+    # Pre-build template choices for the filter_template filter
+    template_choices = None
+    if pp_store:
+        try:
+            templates = pp_store.get_all_templates()
+            template_choices = [{"value": t.id, "label": t.name} for t in templates]
+        except Exception:
+            template_choices = []
+
     responses = []
     for filter_id, filter_cls in all_filters.items():
         schema = filter_cls.get_options_schema()
+        opt_schemas = []
+        for opt in schema:
+            choices = opt.choices
+            # Enrich filter_template choices with current template list
+            if filter_id == "filter_template" and opt.key == "template_id" and template_choices is not None:
+                choices = template_choices
+            opt_schemas.append(FilterOptionDefSchema(
+                key=opt.key,
+                label=opt.label,
+                type=opt.option_type,
+                default=opt.default,
+                min_value=opt.min_value,
+                max_value=opt.max_value,
+                step=opt.step,
+                choices=choices,
+            ))
         responses.append(FilterTypeResponse(
             filter_id=filter_cls.filter_id,
             filter_name=filter_cls.filter_name,
-            options_schema=[
-                FilterOptionDefSchema(
-                    key=opt.key,
-                    label=opt.label,
-                    type=opt.option_type,
-                    default=opt.default,
-                    min_value=opt.min_value,
-                    max_value=opt.max_value,
-                    step=opt.step,
-                )
-                for opt in schema
-            ],
+            options_schema=opt_schemas,
         ))
     return FilterTypeListResponse(filters=responses, count=len(responses))