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/svc/service_context.go

@@ -305,10 +305,11 @@ func newImageGenerator(c config.Config) piagent.ImageGenerator {
 	opts := c.Agent.Image
 	apiKey := resolveImageAPIKey(opts)
 	client := piagent.NewImageClient(piagent.ImageClientOptions{
-		BaseURL:             opts.BaseURL,
-		APIKey:              apiKey,
-		InputImageTransport: opts.InputImageTransport,
-		TimeoutSeconds:      opts.TimeoutSeconds,
+		BaseURL:                      opts.BaseURL,
+		APIKey:                       apiKey,
+		InputImageTransport:          opts.InputImageTransport,
+		TimeoutSeconds:               opts.TimeoutSeconds,
+		ResponseHeaderTimeoutSeconds: opts.ResponseHeaderTimeoutSeconds,
 	})
 	if client == nil {
 		return nil