fix(proxy): wire LLM latency metrics into the streaming response path

NewLLMMetricsCapture/ResponseWriterWrapper had zero call sites, so
llm_ttft_seconds, llm_itl_seconds and llm_tokens_total were exported as
HELP/TYPE stubs with no samples. These are the only per-request LLM latency
series covering the Ollama models (ornith:35b, qwen2.5:3b-instruct), which
expose no /metrics of their own.

Wire the wrapper around the response writer in ServeHTTP for model-routed
requests, and delete the unused io.WriteCloser duplicate that made the gap
hard to spot (two near-identical implementations, neither constructed).

Correctness details:
- Only SSE responses are measured. A non-streaming completion is written in
  one Write, which would otherwise be recorded as a TTFT sample equal to the
  whole request latency with no ITL samples.
- requestStart is captured at the top of ServeHTTP so TTFT measures from
  request acceptance, not from when the upstream response is copied.
- Token count skips the `data: [DONE]` terminator.
- Unwrap() keeps http.ResponseController (Flush/Hijack/deadlines) working
  through the wrapper, so SSE unbuffering (issue #33) is unaffected.

Tests: unit coverage for SSE vs non-streaming, wrapper transparency, and an
end-to-end handler test that fails if the wrapper is ever wired nowhere again.

internal/proxy/proxy.go

@@ -314,6 +314,10 @@ func writeProblemDetail(w http.ResponseWriter, status int, problemType, title, d
 
 // ServeHTTP implements http.Handler.
 func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	// Captured before any work so TTFT measures from the moment the gateway
+	// accepted the request, not from when the upstream response is copied.
+	requestStart := time.Now()
+
 	if h.handleAuthEndpoint(w, r) {
 		return
 	}
@@ -358,7 +362,14 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	// For streaming responses (SSE, chunked), disable buffering. Issue #33.
 	w.Header().Set("X-Accel-Buffering", "no")
 
-	proxy.ServeHTTP(w, r)
+	// Capture per-request LLM latency metrics (TTFT/ITL/tokens) while the
+	// response is copied from the upstream. Inert for non-SSE responses.
+	var respWriter http.ResponseWriter = w
+	if route.ModelName != "" {
+		respWriter = NewResponseWriterWrapper(w, route.ModelName, observability.Global(), requestStart)
+	}
+
+	proxy.ServeHTTP(respWriter, r)
 }