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/app/api/audiobooks/request-with-torrent/route.ts

@@ -57,7 +57,40 @@ export async function POST(request: NextRequest) {
       const body = await req.json();
       const { audiobook, torrent } = RequestWithTorrentSchema.parse(body);
 
-      // Check if audiobook is already available in Plex library
+      // First check: Is there an existing request in 'downloaded' or 'available' status?
+      // This catches the gap where files are organized but Plex hasn't scanned yet
+      const existingActiveRequest = await prisma.request.findFirst({
+        where: {
+          audiobook: {
+            audibleAsin: audiobook.asin,
+          },
+          status: { in: ['downloaded', 'available'] },
+          deletedAt: null,
+        },
+        include: {
+          user: { select: { plexUsername: true } },
+        },
+      });
+
+      if (existingActiveRequest) {
+        const status = existingActiveRequest.status;
+        const isOwnRequest = existingActiveRequest.userId === req.user.id;
+
+        return NextResponse.json(
+          {
+            error: status === 'available' ? 'AlreadyAvailable' : 'BeingProcessed',
+            message: status === 'available'
+              ? 'This audiobook is already available in your Plex library'
+              : 'This audiobook is being processed and will be available soon',
+            requestStatus: status,
+            isOwnRequest,
+            requestedBy: existingActiveRequest.user?.plexUsername,
+          },
+          { status: 409 }
+        );
+      }
+
+      // Second check: Is audiobook already in Plex library? (fallback for non-requested books)
       const plexMatch = await findPlexMatch({
         asin: audiobook.asin,
         title: audiobook.title,