Bug Description

SlackWeeklyLeaderboardTask sends leaderboard notifications to a global Slack channel instead of using workspace-specific notification settings.

Current Behavior

File: leaderboard/tasks/SlackWeeklyLeaderboardTask.java:191-192

// CURRENT: Uses global properties for ALL workspaces
String team = leaderboardProperties.notification().team();
String channelId = leaderboardProperties.notification().channelId();

for (Workspace workspace : workspaces) {
    // ... builds leaderboard ...
    slackMessageService.sendMessage(channelId, blocks, "...");  // Same channel for everyone!
}

Expected Behavior

for (Workspace workspace : workspaces) {
    // Skip workspaces with notifications disabled
    if (!Boolean.TRUE.equals(workspace.getLeaderboardNotificationEnabled())) {
        continue;
    }
    
    // Use workspace-specific settings
    String team = workspace.getLeaderboardNotificationTeam();
    String channelId = workspace.getLeaderboardNotificationChannelId();
    
    if (channelId == null || channelId.isBlank()) {
        log.warn("Skipping workspace {}: no notification channel configured", workspace.getSlug());
        continue;
    }
    
    // ... build and send leaderboard ...
}

Impact

• All workspaces send to the same global Slack channel
• Workspace-specific leaderboardNotificationChannelId is never used
• Workspace-specific leaderboardNotificationTeam is never used
• Workspace-specific leaderboardNotificationEnabled is never checked
• Multi-tenant Slack notifications are broken

Workspace Entity Fields (Exist But Unused)

// In Workspace.java - these fields exist but are ignored!
private Boolean leaderboardNotificationEnabled;
private String leaderboardNotificationTeam;
private String leaderboardNotificationChannelId;

Additional Issue: SlackAppConfig Singleton

SlackAppConfig.java creates a singleton Slack App using global env vars. For true multi-tenant support, we need per-workspace Slack clients using workspace.getSlackToken().

Fix Approach

1. Quick fix: Update SlackWeeklyLeaderboardTask to use workspace fields
2. Full fix: Also update SlackAppConfig to support per-workspace Slack tokens

Related Issues

• Parent epic: feat(epic): workspace-scoped notification & multi-channel leaderboard Slack configuration #480
• feat(application-server): workspace notification settings persistence & API #481 (notification persistence)
• feat(application-server): Slack multi-channel dispatcher & test-send endpoint #483 (multi-channel dispatcher)

Acceptance Criteria

• Task checks workspace.getLeaderboardNotificationEnabled() before sending
• Task uses workspace.getLeaderboardNotificationChannelId() per workspace
• Task uses workspace.getLeaderboardNotificationTeam() per workspace
• Workspaces without channel configured are skipped with warning log
• Unit test verifies per-workspace channel usage