docs: Standardize all IndieAuth spec references to W3C URL

- Updated 42 references from indieauth.spec.indieweb.org to www.w3.org/TR/indieauth
- Ensures consistency across all documentation
- Points to the authoritative W3C specification
- No functional changes, documentation update only

Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -155,7 +155,7 @@ See [docs/architecture/](docs/architecture/) for complete documentation.
 
 StarPunk implements:
 - [Micropub](https://micropub.spec.indieweb.org/) - Publishing API
-- [IndieAuth](https://indieauth.spec.indieweb.org/) - Authentication
+- [IndieAuth](https://www.w3.org/TR/indieauth/) - Authentication
 - [Microformats2](http://microformats.org/) - Semantic HTML markup
 - [RSS 2.0](https://www.rssboard.org/rss-specification) - Feed syndication