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>

docs/architecture/overview.md

@@ -1123,7 +1123,7 @@ The architecture is successful if it enables:
 
 ### External Standards
 - [IndieWeb](https://indieweb.org/)
-- [IndieAuth Spec](https://indieauth.spec.indieweb.org/)
+- [IndieAuth Spec](https://www.w3.org/TR/indieauth/)
 - [Micropub Spec](https://micropub.spec.indieweb.org/)
 - [Microformats2](http://microformats.org/wiki/h-entry)
 - [RSS 2.0](https://www.rssboard.org/rss-specification)