Add LED skip start/end, rename standby_interval to keepalive_interval, remove migrations

LED skip: set first N and last M LEDs to black on a target. Color sources
(static, gradient, effect, color cycle) render across only the active
(non-skipped) LEDs. Processor pads with blacks before sending to device.

Rename standby_interval → keepalive_interval across all Python, API
schemas, and JS. from_dict falls back to old key for existing configs.

Remove legacy migration functions (_migrate_devices_to_targets,
_migrate_targets_to_color_strips) and legacy fields from target model.

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

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

@@ -373,6 +373,10 @@
   "targets.interpolation.dominant": "Dominant",
   "targets.smoothing": "Smoothing:",
   "targets.smoothing.hint": "Temporal blending between frames (0=none, 1=full). Reduces flicker.",
+  "targets.led_skip": "LED Skip:",
+  "targets.led_skip.hint": "Number of LEDs at the start and end of the strip to keep black. Color sources will render only across the active (non-skipped) LEDs.",
+  "targets.led_skip_start": "Start:",
+  "targets.led_skip_end": "End:",
   "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",

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

@@ -373,6 +373,10 @@
   "targets.interpolation.dominant": "Доминантный",
   "targets.smoothing": "Сглаживание:",
   "targets.smoothing.hint": "Временное смешивание между кадрами (0=нет, 1=полное). Уменьшает мерцание.",
+  "targets.led_skip": "Пропуск LED:",
+  "targets.led_skip.hint": "Количество светодиодов в начале и конце ленты, которые остаются чёрными. Источники цвета будут рендериться только на активных (непропущенных) LED.",
+  "targets.led_skip_start": "Начало:",
+  "targets.led_skip_end": "Конец:",
   "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",