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-022-auth-route-prefix-fix.md

@@ -165,7 +165,7 @@ After implementation:
 5. Test full IndieAuth flow with real provider
 
 ## References
-- [IndieAuth Specification](https://indieauth.spec.indieweb.org/) - Section on redirect URIs
+- [IndieAuth Specification](https://www.w3.org/TR/indieauth/) - Section on redirect URIs
 - [OAuth 2.0 RFC 6749](https://tools.ietf.org/html/rfc6749) - Section 3.1.2 on redirection endpoints
 - [RESTful API Design](https://restfulapi.net/resource-naming/) - URL naming conventions
 - Current implementation: `/home/phil/Projects/starpunk/starpunk/routes/auth.py`, `/home/phil/Projects/starpunk/starpunk/auth.py`