docs: remove English translations from journals ...

- Updated 20260223-0013 and 20260223-0015 to be Japanese only.
- Standardized journal format across recent entries.

feat: add MIME type support to documents ...

- Added 'mime' column to 'documents' table in db.rs.
- Implemented automatic MIME detection in mcp.rs 'add_item_text' tool.
- Updated search and retrieval tools in mcp.rs to return 'mime' information.
- Refreshed 03_database_specification.md to reflect the new column.
- Recorded work in journal 20260223-0015.