fix: render notion soft line breaks

[luandro]

Summary

• add restoreSoftLineBreaks to translate Notion soft breaks into <br />
• wire the helper into markdown export so Shift+Enter survives publishing
• extend unit coverage and mocks for the new helper

Testing

• bunx vitest run scripts/notion-fetch/contentSanitizer.test.ts
• bunx vitest run scripts/notion-fetch/generateBlocks.test.ts

Closes #47

[luandro]

Verification Notes

• restoreSoftLineBreaks() now runs right after markdown sanitization (scripts/notion-fetch/generateBlocks.ts:1550), converting single in-paragraph newlines into <br /> while skipping markdown structures. That preserves Shift+Enter breaks from Notion during export.
• Helper is exported via scripts/notion-fetch/utils.ts so downstream code can share it, and unit coverage exercises normal breaks, double breaks, list markers, fenced code, and Notion Unicode separators (scripts/notion-fetch/contentSanitizer.test.ts).

How to test locally

1. In Notion, insert text with a manual line break (Shift+Enter) on a test page.
2. Run the standard sync that regenerates docs (e.g. bun scripts/notion-fetch with your usual filters).
3. Inspect the regenerated markdown under docs/...; the manual break should appear as <br />, and render the same on the site.
4. Optional: run bunx vitest run scripts/notion-fetch/contentSanitizer.test.ts --runInBand --reporter=dot to see the new helper tests pass.