Add configurable SSEKeepAlive interval (default: 15s) that sends SSE comment lines (`:keepalive`) to maintain connection health. Benefits: - Keeps connections alive through proxies/load balancers that timeout idle connections - Detects stale connections earlier (write failures terminate the handler) - Standard SSE pattern - comments are ignored by compliant clients Configuration: - SSEKeepAlive > 0: send keepalives at specified interval - SSEKeepAlive = 0: use default (15s) - SSEKeepAlive < 0: disable keepalives Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>