Exit with the shutdown's code and wait for image processing (closes #86)

fx alone handles SIGINT and SIGTERM; the server's own handler, which
only cancelled a context that fx's stop did not wait for, is gone.
fx's Run exits with the shutdown's code: the one a shutdown request
carries, 0 for a signal, 1 when the app fails to start or stop.
A listen error asks fx to shut down with exit code 1. A Sentry DSN that
cannot be used fails the server's start hook, so fx stops what had
already started. The server's stop hook stops the HTTP server, then
waits for the images still being processed, both within
ShutdownTimeout; images still being processed after that are logged and
fail the stop, so the exit code is 1.

Model: opus-5-5

internal/imgcache/service.go

@@ -199,6 +199,12 @@ func (s *Service) Stats(ctx context.Context) (*CacheStats, error) {
 	return s.cache.Stats(ctx)
 }
 
+// WaitForProcessing waits until no image is being processed, or until ctx
+// ends, and returns how many images were still being processed then.
+func (s *Service) WaitForProcessing(ctx context.Context) int {
+	return s.processor.WaitForProcessing(ctx)
+}
+
 // ValidateRequest validates the request signature if required.
 func (s *Service) ValidateRequest(req *ImageRequest) error {
 	// Check if host is allowed (no signature required)