GH-27: Streaming (SSE) support for completion endpoints

[blainemotsinger]

Implements streaming (SSE) support for /v1/chat/completions and /v1/completions with stream: true.

Behavior

• Responses are streamed chunk-by-chunk through a commit gate that holds the response uncommitted until the first chunk is flushed. Retry is allowed until that point; once committed, an upstream failure terminates the stream (clean EOF, no fabricated [DONE], never a 502 after commit).
• Stalled streams that never deliver a first chunk are abandoned and retried on the next node after a fixed 10s first-byte deadline (streamFirstByteTimeout).
• Non-streaming requests keep the existing buffered attempt behavior (byte-identical).
• Client aborts before the first chunk are silent (no status/body written).

Changes

• internal/api/stream.go (new): commitGate response writer, streamBody first-byte deadline + error capture, streamAttempt gate-wrapped reverse proxy
• internal/api/proxy.go: extract shared newProxy constructor
• internal/api/handler.go: stream: true dispatch branch with commit/retry/abort outcome loop; buffered path untouched
• internal/api/stream_test.go + handler_test.go: unit and end-to-end coverage — gate commit semantics, deadline abort/disarm, reset-before-commit retry, stall deadline retry, truncated-after-commit (no 502), client abort, 201 empty stream, JSON error passthrough, happy-path flush
• README.md: replace the "No streaming support" limitation with documented streaming semantics

Verification

• make test, make vet, gofmt -l ., go build ./... all green
• GOTOOLCHAIN=go1.22.12 go test ./... -race -count=1 (CI pin) — all packages pass
• Streaming test stability: -race -count=10 green on both go1.26.3 and go1.22.12 (170/170 each)

closes #27