feat: add OpenAI Responses API support (POST /v1/responses)

Add full Responses API endpoint alongside existing Chat Completions,
with identical logging/tracking/cost pipeline.

New:
- internal/models/responses.go — request/response/stream types + ToUsage() bridge
- internal/providers/openai_responses.go — OpenAI Responses/ResponsesStream

Modified:
- provider.go — Responses()+ResponsesStream() added to Provider interface
- helpers.go — BuildOpenAIResponsesBody, parsers, SSE stream reader
- circuit_breaker.go — CB wraps Responses, passthrough for stream
- server.go — POST /v1/responses route + handleResponses handler
- all non-OpenAI providers — stub methods with clear error messages

Logging: ResponsesUsage.ToUsage() bridges to models.Usage, feeding same
logRequest() -> DB insert -> dashboard WS -> client stats -> cost calc
pipeline. No schema or logger changes needed.

internal/providers/gemini.go

@@ -185,6 +185,14 @@ func sizeToGeminiAspectRatio(size string) string {
 	}
 }
 
+func (p *GeminiProvider) Responses(ctx context.Context, req *models.ResponsesRequest) (*models.ResponsesResponse, error) {
+	return nil, fmt.Errorf("responses API not supported by gemini")
+}
+
+func (p *GeminiProvider) ResponsesStream(ctx context.Context, req *models.ResponsesRequest) (<-chan *models.ResponsesStreamChunk, error) {
+	return nil, fmt.Errorf("responses API not supported by gemini")
+}
+
 func (p *GeminiProvider) ChatCompletion(ctx context.Context, req *models.UnifiedRequest) (*models.ChatCompletionResponse, error) {
 	// Gemini mapping
 	var contents []GeminiContent