The dashboard persists per-group/per-topic overrides (enabled,
require_mention, skills, tools, system_prompt, ...) into the channel
instance config, but telegramInstanceConfig had no groups field, so the
payload was dropped on unmarshal. config.TelegramConfig.Groups stayed
nil, resolveTopicConfig fell back to global defaults, and the bot
replied in every forum topic regardless of the configured per-topic
gating.
Map the groups field through the DB-instance factory so per-topic
config takes effect for dashboard-created channels, matching the static
file-config path.
