chore: update architecture and documentation to reflect removal of MongoDB and transition to NAS storage for chapter content

- Updated `.env.example` to remove MongoDB configuration.
- Revised `ARCHITECTURE.md` to reflect changes in data orchestration and storage strategy.
- Removed `CHAPTER_SAVE_DEBUG.md` and `FIXES_APPLIED.md` as they are no longer relevant.
- Updated `CONTRACT.md` to include new error codes.
- Adjusted endpoint documentation in `CROSS_REPO_ENDPOINT_MATRIX.md` and `README.md` to align with the new import flow.
- Removed legacy import scripts and tables from the codebase.

prisma/schema.prisma

@@ -161,7 +161,7 @@ model Comment {
   content   String   @db.Text
   userId    String
   novelId   String
-  chapterId String?  // Có thể bình luận riêng tư cho từng chương (Lưu chapterId từ MongoDB)
+  chapterId String?  // Bình luận theo chương (id chương từ backend / ChapterMeta)
   
   user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)
   novel     Novel    @relation(fields: [novelId], references: [id], onDelete: Cascade)