Simplify CategoryUploadHandler

[River-Qinghao]

Motivation

• Align CategoryUploadHandler implementation with the simpler JournalUploadHandler style to reduce complexity and follow project conventions.
• Remove extra helper methods and state spread across the class to make the handler easier to read and maintain.

Description

• Import sqlite3 and simplify CategoryUploadHandler in Handlers.py to inline table creation and inserts inside pushDataToDb instead of separate helper methods like _create_tables, _get_or_create_category, and _get_or_create_area.
• Keep relational schema with tables journals, categories, areas, journal_categories, journal_areas, and category_areas, and populate them using INSERT OR IGNORE and SELECT within pushDataToDb.
• Preserve behavior to only accept .json files, commit the transaction with conn.commit(), close the connection with conn.close(), and return True on success.

Testing

• No automated tests were executed for this change.

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Codex Task