bug: duplicate root categories — original and Chinese-named entries coexist

[Jing-yilin]

Description

The categories table has duplicate root categories. For example:

• Art (id: Q2F0ZWdvcnktMQ==, no name_zh)
• Art with name_zh=艺术 (separate record)

This results in 29 root categories instead of the expected ~15. The Vertex AI Chinese translation created new category rows instead of updating the name_zh field on existing ones.

Evidence

GET /api/categories
# Returns 197 categories, 29 roots (should be ~15)
# Only 26/197 (13.2%) have name_zh populated

Expected Behavior

Each category should have ONE record with both name and name_zh populated. Root categories should be ~15, not 29.

Fix

1. Merge duplicate category records — update name_zh on originals
2. Delete the duplicate rows
3. Fix the translation code to UPDATE existing categories instead of INSERT