Alert emails ignore the configured sender display name

The settings repository mapper (`toEntity` in `app-settings.repository.mongo.ts`) never returns `systemEmailDisplayName`. `EmailService.sendEmail` reads its transport config from `settingsService.getDBSettings()`, which goes through that mapper, so every real alert email is sent from the bare address. The test-email button passes the form values straight through, so it does show the display name, which makes this easy to miss.

Looks like it was lost in the domain reorg (d022f7d84). One-line fix plus a repository test; PR coming right after this.

Start in app-settings.repository.mongo.ts at the toEntity mapper and trace settingsService.getDBSettings() into EmailService.sendEmail. Restore the missing systemEmailDisplayName mapping, add the repository test mentioned in the issue, and verify that real alert-email settings preserve the configured sender display name.

Written by the indexing model from the issue text.