Get count of active users grouped by language.

Returns: Dictionary with language codes as keys and counts of learners/natives as values.

Example response:

{
    "languages": {
        "spanish": {"learners": 5, "natives": 3},
        "english": {"learners": 8, "natives": 12}
    },
    "total_users": 15,
    "timestamp": "2025-12-09T12:00:00Z"
}

Notes:

• Only shows languages with at least 3 total users (privacy protection)
• Results are cached for 30 seconds to reduce database load
• Uses WebSocket heartbeat tracking to determine active users