Add flag-related email notifications for flagged player and offline opponent

[Copilot]

This change addresses missing player feedback when a user loses on time (flags). It now covers both players:

• the player who flagged receives a direct notification, and

• the opponent can receive an offline notification (after being offline long enough), with a user setting to enable/disable it.

• Flag event now emails the flagged player

  • PlayerVsPlayerGameService.flagByColor(...) triggers a mail notification after a successful flag update.
  • The recipient is resolved from the flagged player's email; if no email is available, this notification is skipped.

• Flag event can now email the other player when offline

  • Added offline-threshold-based opponent notification logic (aligned with existing offline notification behavior).
  • Added MailService.sendOpponentFlaggedWhileOffline(...) and a new template:
    • mail_templates/opponent_flagged_while_offline.html

• New notification preference in /user/settings

  • Added a checkbox for opponent-flagged-offline notifications.
  • Wired end-to-end through notifications API/DTO/DAO and persistence.

• Data model and migration updates

  • Added email_notification_enabled_opponent_flagged to user notification settings.
  • Updated Liquibase changelogs accordingly.

• Updated existing flagged mail API/template

  • MailService.sendUserFlaggedNotification(...) accepts recipient email and sends directly to the flagged player.
  • mail_templates/user_flagged_notification.html is user-facing and includes username, game ID, and game link.

• Template and settings coverage

  • Updated MailRendererTest for user_flagged_notification.
  • Added MailRendererTest case for opponent_flagged_while_offline.
  • Updated UserDaoServiceTest assertions for the new notification setting.

mailService.sendUserFlaggedNotification(
    recipient = email,
    gameId = gameId,
    username = userCache.fetchUsernameOrDefault(userId),
)

[benckx]

@copilot it's not the admin that should be notified but the player who lost by flagging

[benckx]

@copilot actually the other player too. But they have to be offline for a while (like the other mail notifications). And there must be a checkbox in the /user/settings to enable or disable this notification.

[Copilot]

@copilot it's not the admin that should be notified but the player who lost by flagging

Updated in commit 3939ca0: flagged notifications are now sent to the player who flagged (using their email), not to admin.

[Copilot]

@copilot actually the other player too. But they have to be offline for a while (like the other mail notifications). And there must be a checkbox in the /user/settings to enable or disable this notifi...

Implemented in commit 781ec00: the other player is now notified too, but only when they have been offline long enough, and a new /user/settings checkbox controls this (opponent flagged while offline). Screenshot: /tmp/flagged-notification-setting.png

[benckx]

@copilot resolve the merge conflicts in this pull request

[benckx]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Done in commit e04b346: merged latest origin/master into this PR branch and resolved the Liquibase changelog conflicts.