feat: replace brittle DOM scraper with RSS-first parser and semantic HTML fallback

- Prioritize NASA official RSS feed (https://science.nasa.gov/feed/apod-basic/)
- Semantic fallback parser for science.nasa.gov/apod/ without fragile class chains
- Delete legacy 1995-era center tag scraping logic
- Universal explanation prefix and boilerplate stripping
- Add validation guard in fetchAPOD preventing empty explanation emails
- Add comprehensive test coverage for RSS, date filtering, and fallback flows

Author: gtracy

.gitignore

@@ -155,4 +155,5 @@ email_preview.html
 
 # Python venv
 venv/
-venv311/
+venv311/
+.gstack/