New admin-only server function that re-renders content_html and
toc_html for posts from their content_md:
- rebuild_all=false: only processes posts where content_html IS NULL
- rebuild_all=true: processes all non-deleted posts
- Processes rows one at a time to avoid table locks
- Invalidates post list and stats caches after rebuild
- Move posts.rs (891 lines) to posts/ with 11 submodules
- Extract shared tag sync logic into helpers::sync_tags
- Extract tag cleaning into helpers::clean_tags
- All external import paths unchanged via mod.rs re-exports
- Zero behavioral changes, pure structural refactor
