fix(image): add response-header timeout and idempotent retry

Give the image client a configurable ResponseHeaderTimeout (default 4m,
capped at the overall timeout) so a stalled upstream fails fast, and
retry such timeouts when an idempotency key is set so a slow-but-safe
request can recover instead of terminating. Raise the Next.js API proxy
maxDuration to 1200s to accommodate long generations.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/config/config.go

@@ -123,11 +123,12 @@ type AgentConfig struct {
 }
 
 type ImageGenerationConfig struct {
-	BaseURL             string `json:",optional"`
-	APIKey              string `json:",optional"`
-	APIKeyEnv           string `json:",default=GPT_IMAGE_API_KEY"`
-	InputImageTransport string `json:",default=file"`
-	TimeoutSeconds      int    `json:",default=600"`
+	BaseURL                      string `json:",optional"`
+	APIKey                       string `json:",optional"`
+	APIKeyEnv                    string `json:",default=GPT_IMAGE_API_KEY"`
+	InputImageTransport          string `json:",default=file"`
+	TimeoutSeconds               int    `json:",default=600"`
+	ResponseHeaderTimeoutSeconds int    `json:",default=240"`
 }
 
 type TextExtractionConfig struct {