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-021-indieauth-provider-strategy.md

@@ -505,7 +505,7 @@ If there is user demand for a more integrated solution, V2 could add:
 ## References
 
 ### IndieAuth Specifications
-- [IndieAuth Spec](https://indieauth.spec.indieweb.org/) - Official W3C specification
+- [IndieAuth Spec](https://www.w3.org/TR/indieauth/) - Official W3C specification
 - [OAuth 2.0](https://oauth.net/2/) - Underlying OAuth 2.0 foundation
 - [Client Identifier](https://www.oauth.com/oauth2-servers/indieauth/) - How client_id works in IndieAuth