chore(translate): consolidate data-URL placeholder regex into single constant

[luandro]

Background

scripts/notion-translate/translateFrontMatter.ts defines its own inline regex for detecting data-URL placeholder paths:

```ts
// line ~50
//images/data_url_placeholder\d+_.png/
```

A DATA_URL_PLACEHOLDER_REGEX constant already exists elsewhere in the codebase. These two definitions can drift apart silently.

What to do

• Import and reuse the existing DATA_URL_PLACEHOLDER_REGEX constant in translateFrontMatter.ts (or move the canonical definition to constants.ts if it isn't there already)
• Remove the inline duplicate regex
• Add a test or comment that makes the single-source-of-truth relationship explicit