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.

.env.example

@@ -1,6 +1,5 @@
-# PostgreSQL + MongoDB
+# PostgreSQL
 DATABASE_URL=postgresql://reader:reader@localhost:5432/reader
-MONGODB_URI=mongodb://localhost:27017/reader
 
 # Auth / OAuth
 NEXTAUTH_SECRET=replace-with-strong-secret