feat(domain): scan before the PDF check so malware is always reported as infected (refs #192)

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

services/domain/Big.Application/ProvideDocuments.cs

@@ -50,16 +50,17 @@ public sealed class ProvideDocuments(IRegistrationStore store, IWorkflowClient w
             return ProvideDocumentsOutcome.NotFound;
 
         // Only a clean PDF goes any further (S-29, ADR-0036): checked after ownership, so a stranger
-        // learns nothing about the file, and before anything is stored or the wait is completed.
-        if (!command.Content.AsSpan().StartsWith("%PDF-"u8))
-            return ProvideDocumentsOutcome.NotAPdf;
-
+        // learns nothing about the file, and before anything is stored or the wait is completed. Scan
+        // before the PDF check, so malware is reported as malware whatever it claims to be.
         switch (await scanner.ScanAsync(command.Content, ct))
         {
             case ScanVerdict.Infected: return ProvideDocumentsOutcome.Infected;
             case ScanVerdict.Unavailable: return ProvideDocumentsOutcome.ScannerUnavailable;
         }
 
+        if (!command.Content.AsSpan().StartsWith("%PDF-"u8))
+            return ProvideDocumentsOutcome.NotAPdf;
+
         // Store the diploma against the zaak (once it is opened) — the ACL is the only ZGW caller (§8.1).
         if (registration.ZaakUrl is not null)
             await acl.StoreDiplomaAsync(