feat(evidence): allow .exe uploads for Red-team artifacts shared with Blue

The evidence upload extension whitelist blocked .exe entirely, but sharing
a Red team payload binary with Blue for detection analysis is a normal
part of this platform's workflow.

backend/app/services/evidence_service.py

@@ -57,6 +57,8 @@ ALLOWED_EXTENSIONS: frozenset[str] = frozenset({
     ".zip", ".tar", ".gz", ".7z",
     # Literal argument value
     ".har", ".eml", ".msg",
+    # Red team artifacts shared with Blue for detection analysis.
+    ".exe",
 })