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-029-micropub-indieauth-integration.md

@@ -518,8 +518,8 @@ DELETE FROM auth_state WHERE expires_at < datetime('now');
 
 ## References
 
-- [IndieAuth Spec - Token Endpoint](https://indieauth.spec.indieweb.org/#token-endpoint)
-- [IndieAuth Spec - Authorization Code](https://indieauth.spec.indieweb.org/#authorization-code)
+- [IndieAuth Spec - Token Endpoint](https://www.w3.org/TR/indieauth/#token-endpoint)
+- [IndieAuth Spec - Authorization Code](https://www.w3.org/TR/indieauth/#authorization-code)
 - [Micropub Spec - Authentication](https://www.w3.org/TR/micropub/#authentication)
 - [OAuth 2.0 Security Best Practices](https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics)