Mijn aanvragen (B): store document bytes + content endpoint

- StoredDocument gains ContentType + byte[] Content; POST /uploads now captures
  the file bytes (in-memory, reset on restart — POC).
- GET /uploads/{documentId}/content streams the bytes: inline for pdf/image
  (browser preview), attachment otherwise (download). 404 for unknown ids
  (covers the demo-* simulation sentinels, which have no bytes).
- Bytes are never serialized into a JSON response; only this endpoint streams them.
- Tests: content served back with type inline for pdf, 404 for unknown. 56/56 green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/tests/BigRegister.Tests/EndpointTests.cs

@@ -164,6 +164,24 @@ public class EndpointTests(WebApplicationFactory<Program> factory) : IClassFixtu
         Assert.Contains(status.Results, r => r.LocalId == "onbekend" && r.Status == "unknown");
     }
 
+    [Fact]
+    public async Task Upload_content_is_served_back_with_its_type_inline_for_pdf()
+    {
+        var doc = await Upload(Guid.NewGuid().ToString());
+        var res = await _client.GetAsync($"/api/v1/uploads/{doc.DocumentId}/content");
+        res.EnsureSuccessStatusCode();
+        Assert.Equal("application/pdf", res.Content.Headers.ContentType!.MediaType);
+        Assert.Equal(new byte[] { 1, 2, 3 }, await res.Content.ReadAsByteArrayAsync());
+        // pdf/image → inline (no attachment disposition) so the browser previews it
+        Assert.NotEqual("attachment", res.Content.Headers.ContentDisposition?.DispositionType);
+    }
+
+    [Fact]
+    public async Task Upload_content_404_for_unknown_document()
+    {
+        Assert.Equal(HttpStatusCode.NotFound, (await _client.GetAsync("/api/v1/uploads/demo-nope/content")).StatusCode);
+    }
+
     [Fact]
     public async Task Upload_rejects_wrong_type()
     {