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

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
- Paired with ResponseController.Flush() for unbuffered token delivery

**#32 HTTP/2 multiplexing for concurrent streams**
- Enable HTTP/2 in server config via http2.ConfigureServer()
- Increase MaxConnsPerHost from default (2) to 10
- ForceAttemptHTTP2 on outbound Transport for upstream connections
- 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
- Upstream Transport respects backpressure when clients read slowly

**Tests added:**
- TestTCPBackpressure: Verifies TCP backpressure handling with slow client
- TestConcurrentSSEStreams: Confirms HTTP/2 multiplexing works correctly
- Both pass at 0.11s and 0.06s respectively

Fixes all three streaming performance issues in one coherent change.

..bfg-report/2026-09-13/11-02-26/cache-stats.txt

@@ -0,0 +1,4 @@
+(apply,CacheStats{hitCount=337, missCount=199, loadSuccessCount=199, loadExceptionCount=0, totalLoadTime=581291927, evictionCount=0})
+(tree,CacheStats{hitCount=986, missCount=352, loadSuccessCount=299, loadExceptionCount=0, totalLoadTime=821650758, evictionCount=0})
+(commit,CacheStats{hitCount=108, missCount=107, loadSuccessCount=107, loadExceptionCount=0, totalLoadTime=78983052, evictionCount=0})
+(tag,CacheStats{hitCount=0, missCount=2, loadSuccessCount=2, loadExceptionCount=0, totalLoadTime=319542, evictionCount=0})