feat: add packet_buffer_init_video config parameter

[Rajiv91]

Summary

• Add new YAML config parameter packet_buffer_init_video that sets the initial size of the SFU's circular packet buffer for video, independently of packet_buffer_size_video (max capacity)
• Under degraded networks (WiFi/5G congestion at trade shows), H.264 packets arrive in bursts of 1,000-2,500 that exceed the default initial bucket size (300). The PPS-based growth logic doesn't detect the need because it uses average PPS, not peak burst size
• Falls back to InitPacketBufferSizeVideo (300) if not set — zero impact on existing deployments

For a more detailed analysis, please go to number 4 on the list:
https://www.notion.so/irisai/Low-quality-network-investigation-33645056d4d380d4b1e5f3e825077d9d

Cross-repo PRs

• Paired with tryiris-ai/platform branch fix/livekit-packet-buffer-size (passes packet_buffer_init_video: 2400 in the encoder's LiveKit config)

Test plan

• Validated with 10+ hours of active H.264 streaming under aggressive network shaping (25-30 Mbps, 50-75ms delay, 0.5-0.75% loss)
• Zero packetNotFoundCount errors (vs 134,000+ without the fix)
• VP8/NDI streams verified unaffected (zero errors in all sessions)
• Burst peaks of up to 2,487 packets absorbed within 2,400 capacity

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