Implement leave announcement for banned members and enhance moderation notifications

- Added a `leaveAnnounceBan` field to the `WelcomeConfig` model, allowing for optional announcements in the leave channel when a member is banned.
- Updated the leave handling logic to check for bans and send a notice in the leave channel if the `leaveAnnounceBan` toggle is enabled.
- Enhanced moderation actions (kick and ban) to notify the affected user via DM with the reason for the action.
- Introduced a new `notifyModerationTargetDm` function to streamline DM notifications for moderation actions.
- Updated the WebUI to include a toggle for the `leaveAnnounceBan` feature, improving user configurability.
- Improved localization for new messages related to moderation notifications and leave announcements in both English and German.
- Documented changes in phase tracking to reflect the new leave announcement feature and moderation notification enhancements.

apps/bot/prisma/migrations/20260725190000_leave_announce_ban/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "WelcomeConfig" ADD COLUMN "leaveAnnounceBan" BOOLEAN NOT NULL DEFAULT false;

apps/bot/prisma/schema.prisma

@@ -254,6 +254,8 @@ model WelcomeConfig {
   leaveContent     String?
   leaveEmbed       Json?
   leaveComponents  Json?
+  /// When true, leave channel also posts a notice if the member was banned.
+  leaveAnnounceBan Boolean  @default(false)
   welcomeDmEnabled Boolean  @default(false)
   welcomeDmContent String?
   userAutoroleIds  String[] @default([])