chore: remove legacy tables and series table scripts

- Removed mongoose dependency from package-lock.json.
- Deleted legacy import tables: ImportCandidateChapter, AssetNovelMapping, ImportJob, ImportSession, SourceAsset via new script `drop_legacy_import_tables.py`.
- Added script `drop_series_table.py` to drop the Series table.

Co-authored-by: Copilot <copilot@github.com>

.gitignore

@@ -6,6 +6,7 @@ __pycache__/
 .venv/
 dist/
 build/
+node_modules/
 
 .DS_Store
 .env
@@ -17,3 +18,4 @@ test_*.py
 data/epub-source/*
 data/content/*
 data/nas-content/*
+node_modules