feat(network): SSE optimization for local LLM streaming (#31 #32 #33) (#26)

Addresses three critical network issues for LLM streaming performance:

**#33 Disable proxy buffering for SSE**
- Add X-Accel-Buffering: no header to response
- Tells nginx/Ingress to stream events immediately instead of buffering

**#32 HTTP/2 multiplexing for concurrent streams**
- Enable HTTP/2 in server config via http2.ConfigureServer()
- Increase MaxConnsPerHost to 10 for better concurrency
- Allows multiple concurrent LLM requests without blocking

**#31 TCP backpressure for streaming LLM responses**
- Set TCP_NODELAY on dialer to disable Nagle's algorithm
- Reduces latency by sending small packets immediately
- Critical for low TTFT (time-to-first-token) under load

**Tests added:**
- TestTCPBackpressure: Verifies TCP backpressure handling with slow client
- TestConcurrentSSEStreams: Confirms HTTP/2 multiplexing works correctly

---------

Co-authored-by: poison <[email protected]>
Reviewed-on: #26
Co-authored-by: poimen <[email protected]>

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