feat: add whitewind (com.whtwnd.blog.entry) indexing support
- add com.whtwnd.blog.entry to tap collection filters and document routing
- add content-as-string fallback in extractor (whitewind stores markdown in content field)
- add visibility filter to skip non-public whitewind entries
- add whitewind platform to frontend (filter button, URL pattern, config)
- add stats link to header
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>