⚡ Optimize Redis usage in player view broadcast

[WeixuanZ]

💡 What:
Optimized get_player_view and broadcast_filtered_states to batch presence checks.

• Added get_all_player_presence method to GameService to fetch presence for all players in one go.
• Modified get_player_view to accept an optional presence_map.
• Updated broadcast_filtered_states in backend/app/api/routers/rooms.py to fetch presence once and pass it down.

🎯 Why:
Previously, broadcasting game state to N players resulted in N separate calls to get_player_view, each triggering a Redis mget for presence. This caused N round-trips to Redis.
With this change, presence for all players is fetched once per broadcast, reducing Redis calls from N (per connected player) to 1.

📊 Measured Improvement:
Benchmark with N=50 players:

• Baseline: 50 Redis MGET calls, ~0.10s execution time.
• Optimized: 1 Redis MGET call, ~0.036s execution time.
• Improvement: ~3x speedup in this specific hot path and 98% reduction in Redis calls during broadcast.

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PR created automatically by Jules for task 8902105928956179380 started by @WeixuanZ

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