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/decisions/ADR-028-micropub-implementation.md

@@ -204,7 +204,7 @@ We will implement a **minimal but complete Micropub server** for V1, focusing on
 ## References
 
 - [W3C Micropub Specification](https://www.w3.org/TR/micropub/)
-- [IndieAuth Specification](https://indieauth.spec.indieweb.org/)
+- [IndieAuth Specification](https://www.w3.org/TR/indieauth/)
 - [OAuth 2.0 Bearer Token Usage](https://tools.ietf.org/html/rfc6750)
 - [Micropub Rocks Validator](https://micropub.rocks/)