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/max_concurrent_processing_internal_test.go

@@ -300,3 +300,69 @@ func TestProcessReleasesSlotOnError(t *testing.T) {
 		})
 	}
 }
+
+// TestWaitForProcessing holds a processing slot with a Process call that
+// cannot finish reading its input. WaitForProcessing must report that image
+// when its context ends first, wait for it otherwise, return 0 once it has
+// finished, and give back the slots it took while waiting.
+func TestWaitForProcessing(t *testing.T) {
+	t.Parallel()
+
+	proc := New(Params{MaxConcurrentProcessing: 2})
+
+	gate := make(chan struct{})
+	entered := make(chan struct{}, 1)
+	results := make(chan error, 1)
+
+	openGate := sync.OnceFunc(func() { close(gate) })
+	t.Cleanup(openGate)
+
+	processInBackground(proc, &gatedReader{
+		data: bytes.NewReader(createTestJPEG(t, 10, 10)), gate: gate,
+		entered: entered, counter: &readingCounter{},
+	}, results)
+	waitForEntries(t, entered, 1)
+
+	ctx, cancel := context.WithTimeout(t.Context(), 100*time.Millisecond)
+	defer cancel()
+
+	stillProcessing := proc.WaitForProcessing(ctx)
+	t.Logf("WaitForProcessing() after its context ended: %d", stillProcessing)
+
+	if stillProcessing != 1 {
+		t.Errorf("WaitForProcessing() after its context ended = %d, want 1",
+			stillProcessing)
+	}
+
+	waited := make(chan int, 1)
+
+	go func() { waited <- proc.WaitForProcessing(t.Context()) }()
+
+	select {
+	case got := <-waited:
+		t.Fatalf("WaitForProcessing() = %d while an image was being processed",
+			got)
+	case <-time.After(100 * time.Millisecond):
+	}
+
+	openGate()
+
+	err := <-results
+	if err != nil {
+		t.Errorf("Process() error = %v, want nil", err)
+	}
+
+	select {
+	case got := <-waited:
+		if got != 0 {
+			t.Errorf("WaitForProcessing() once processing finished = %d, want 0",
+				got)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("WaitForProcessing() did not return once processing finished")
+	}
+
+	if held := len(proc.processingSemaphore); held != 0 {
+		t.Errorf("%d slots still held after WaitForProcessing() returned", held)
+	}
+}