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/imageprocessor/imageprocessor.go

@@ -331,6 +331,31 @@ func FormatToMIME(format Format) string {
 	}
 }
 
+// WaitForProcessing waits until no image is being processed, or until ctx
+// ends, and returns how many images were still being processed then. It
+// waits by taking every slot in processingSemaphore as it frees up, so no
+// new image starts meanwhile, and gives them all back before it returns.
+func (p *ImageProcessor) WaitForProcessing(ctx context.Context) int {
+	taken := 0
+
+	defer func() {
+		for range taken {
+			<-p.processingSemaphore
+		}
+	}()
+
+	for taken < cap(p.processingSemaphore) {
+		select {
+		case p.processingSemaphore <- struct{}{}:
+			taken++
+		case <-ctx.Done():
+			return len(p.processingSemaphore) - taken
+		}
+	}
+
+	return 0
+}
+
 // acquireSlot takes a slot in processingSemaphore, waiting at most
 // processingWaitTimeout for one to free up, and returns the func that gives
 // it back. A free slot is taken even when ctx has ended; only the wait for