Fix LessonFile duplication: upsert on upload, delete S3 on remove

POST /api/admin/lesson-files now checks for an existing record with the
same (lessonId, name) before uploading — replaces it (old S3 object
deleted) instead of always creating a new one. Previously every save
cycle accumulated an extra copy; 1183 duplicates occupying 6.5 GiB were
found and cleaned up.

DELETE now receives the file URL and extracts the S3 key from it, so
manual deletion actually removes the object from storage.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/admin/lesson-files-manager.tsx

@@ -36,12 +36,12 @@ export function LessonFilesManager({ lessonId, initialFiles }: { lessonId: strin
     e.target.value = "";
   }
 
-  async function handleDelete(fileId: string) {
+  async function handleDelete(fileId: string, url: string) {
     if (!confirm("Удалить файл?")) return;
     await fetch("/api/admin/lesson-files", {
       method: "DELETE",
       headers: { "Content-Type": "application/json" },
-      body: JSON.stringify({ fileId }),
+      body: JSON.stringify({ fileId, url }),
     });
     setFiles((prev) => prev.filter((f) => f.id !== fileId));
   }
@@ -122,7 +122,7 @@ export function LessonFilesManager({ lessonId, initialFiles }: { lessonId: strin
               </span>
               <button
                 type="button"
-                onClick={() => handleDelete(f.id)}
+                onClick={() => handleDelete(f.id, f.url)}
                 className="text-xs"
                 style={{ color: "oklch(0.577 0.245 27.325)" }}
               >