feat: send email notifications for missed messages

- New cron job in cluster
- Handle presence and other concerns
- Toggle in app to disable email notifications
- Handles other edge cases such as cooldown period
- Simple html email with simple message

Closes #117

go/internal/pusher/redis_bridge.go

@@ -143,6 +143,41 @@ func (rb *RedisBridge) GetPresence(ctx context.Context, channelIDs []string) (ma
 	return result, nil
 }
 
+// GetAllConnectedHumanIDs scans all channel connection hashes in Redis and returns
+// the deduplicated set of all humanIDs that have at least one active connection.
+func (rb *RedisBridge) GetAllConnectedHumanIDs(ctx context.Context) ([]string, error) {
+	allHumanIDs := make(map[string]bool)
+	var cursor uint64
+
+	for {
+		keys, nextCursor, err := rb.client.Scan(ctx, cursor, channelConnsPrefix+"*"+channelConnsSuffix, 100).Result()
+		if err != nil {
+			return nil, fmt.Errorf("failed to scan channel keys: %w", err)
+		}
+
+		for _, key := range keys {
+			members, err := rb.client.HVals(ctx, key).Result()
+			if err != nil && err != redis.Nil {
+				continue
+			}
+			for _, humanID := range members {
+				allHumanIDs[humanID] = true
+			}
+		}
+
+		cursor = nextCursor
+		if cursor == 0 {
+			break
+		}
+	}
+
+	result := make([]string, 0, len(allHumanIDs))
+	for id := range allHumanIDs {
+		result = append(result, id)
+	}
+	return result, nil
+}
+
 // --- Pub/Sub listener (runs in its own goroutine) ---
 
 // Listen subscribes to Redis Pub/Sub and forwards events to the local hub.