fix: reject a malformed reenqueue body instead of running the full scope

Option<Json<T>> collapses a malformed body to None, so a bad reenqueue body
silently ran the default full "All" scope (audit). Parse raw bytes: empty =
default All, malformed = 422. Tested both.

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

backend/src/api/admin/analysis.rs

@@ -328,10 +328,21 @@ fn ensure_enabled(state: &AppState) -> AppResult<()> {
 async fn reenqueue(
     State(state): State<AppState>,
     admin: RequireAdmin,
-    body: Option<Json<ReenqueueBody>>,
+    raw: axum::body::Bytes,
 ) -> AppResult<Json<ReenqueueResponse>> {
     ensure_enabled(&state)?;
-    let body = body.map(|b| b.0).unwrap_or_default();
+    // An ABSENT/empty body means the default "All" scope. A PRESENT body must be
+    // valid JSON — `Option<Json<T>>` would silently collapse a malformed body to
+    // None and run the full-library default the caller never intended, so parse
+    // the raw bytes ourselves and 422 on a real parse error.
+    let body: ReenqueueBody = if raw.iter().all(u8::is_ascii_whitespace) {
+        ReenqueueBody::default()
+    } else {
+        serde_json::from_slice(&raw).map_err(|e| AppError::ValidationFailed {
+            message: "reenqueue body is not valid JSON".into(),
+            details: json!({ "body": e.to_string() }),
+        })?
+    };
 
     // Resolve the scope. manga_id and chapter_id are mutually exclusive;
     // an unknown target is a 404 rather than a silent zero-enqueue.