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/static/locales/en.json

@@ -297,6 +297,7 @@
   "filters.auto_crop": "Auto Crop",
   "filters.flip": "Flip",
   "filters.color_correction": "Color Correction",
+  "filters.filter_template": "Filter Template",
   "postprocessing.description_label": "Description (optional):",
   "postprocessing.description_placeholder": "Describe this template...",
   "postprocessing.created": "Template created successfully",
@@ -372,8 +373,8 @@
   "targets.interpolation.dominant": "Dominant",
   "targets.smoothing": "Smoothing:",
   "targets.smoothing.hint": "Temporal blending between frames (0=none, 1=full). Reduces flicker.",
-  "targets.standby_interval": "Standby Interval:",
-  "targets.standby_interval.hint": "How often to resend the last frame when the screen is static, keeping the device in live mode (0.5-5.0s)",
+  "targets.keepalive_interval": "Keep Alive Interval:",
+  "targets.keepalive_interval.hint": "How often to resend the last frame when the source is static, keeping the device in live mode (0.5-5.0s)",
   "targets.created": "Target created successfully",
   "targets.updated": "Target updated successfully",
   "targets.deleted": "Target deleted successfully",

server/src/wled_controller/static/locales/ru.json

@@ -297,6 +297,7 @@
   "filters.auto_crop": "Авто Обрезка",
   "filters.flip": "Отражение",
   "filters.color_correction": "Цветокоррекция",
+  "filters.filter_template": "Шаблон фильтров",
   "postprocessing.description_label": "Описание (необязательно):",
   "postprocessing.description_placeholder": "Опишите этот шаблон...",
   "postprocessing.created": "Шаблон успешно создан",
@@ -372,8 +373,8 @@
   "targets.interpolation.dominant": "Доминантный",
   "targets.smoothing": "Сглаживание:",
   "targets.smoothing.hint": "Временное смешивание между кадрами (0=нет, 1=полное). Уменьшает мерцание.",
-  "targets.standby_interval": "Интервал ожидания:",
-  "targets.standby_interval.hint": "Как часто повторно отправлять последний кадр при статичном экране для удержания устройства в режиме live (0.5-5.0с)",
+  "targets.keepalive_interval": "Интервал поддержания связи:",
+  "targets.keepalive_interval.hint": "Как часто повторно отправлять последний кадр при статичном источнике для удержания устройства в режиме live (0.5-5.0с)",
   "targets.created": "Цель успешно создана",
   "targets.updated": "Цель успешно обновлена",
   "targets.deleted": "Цель успешно удалена",