start improving dbreaders

[jepegit]

Note

Introduce BaseSimpleDbReader and switch Excel/SQL readers to it; add JSON DB reader scaffold; update batch engine to accept reader type strings and streamline field retrieval.

• DB Reader Abstractions:
  • Add BaseSimpleDbReader with concrete per-cell accessors and from_batch contract.
  • Extend BaseDbReader with from_batch(...) signature.
  • Switch dbreader.Reader and sql_dbreader.SQLReader to inherit from BaseSimpleDbReader.
• New JSON Reader:
  • Add readers/json_dbreader.py with BaseJSONReader and BattBaseJSONReader (stub from_batch).
• Batch Engine (utils/batch_tools/engines.py):
  • Allow simple_db_engine to accept reader by string (e.g., "simple_excel_reader", "batbase_json_reader").
  • Refactor journal field population using a loop; add fallback for missing file_name_indicator.
• Misc:
  • Minor docstrings/type hints and interface cleanups in readers/core.py.

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