Add e-book sidecar integration and improve request handling

Introduces optional e-book sidecar downloads from Anna's Archive, including admin UI, settings API, FlareSolverr integration, and documentation. Enhances request creation logic to prevent duplicate downloads by checking for 'downloaded' and 'available' statuses, updates UI to reflect processing state, and adds SABnzbd support to download and cleanup flows. Also updates ranking algorithm documentation and improves cache invalidation for recent requests.

src/components/audiobooks/AudiobookDetailsModal.tsx

@@ -364,6 +364,7 @@ export function AudiobookDetailsModal({
                   'searching',
                   'downloading',
                   'processing',
+                  'downloaded',
                   'awaiting_import',
                 ];
                 if (
@@ -371,10 +372,15 @@ export function AudiobookDetailsModal({
                   requestStatus &&
                   inProgressStatuses.includes(requestStatus)
                 ) {
-                  // Show who requested it
-                  const buttonText = requestedByUsername
-                    ? `Requested by ${requestedByUsername}`
-                    : 'Already Requested';
+                  // Special text for 'downloaded' status (waiting for Plex scan)
+                  let buttonText;
+                  if (requestStatus === 'downloaded') {
+                    buttonText = 'Processing...';
+                  } else {
+                    buttonText = requestedByUsername
+                      ? `Requested by ${requestedByUsername}`
+                      : 'Already Requested';
+                  }
 
                   return (
                     <div className="flex-1">