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-018-indieauth-detailed-logging.md

@@ -819,7 +819,7 @@ LOG_LEVEL=DEBUG
 - [Python Logging Documentation](https://docs.python.org/3/library/logging.html)
 - [OWASP Logging Cheat Sheet](https://cheatsheetseries.owasp.org/cheatsheets/Logging_Cheat_Sheet.html)
 - [OAuth 2.0 Security Best Current Practice](https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics)
-- [IndieAuth Specification](https://indieauth.spec.indieweb.org/)
+- [IndieAuth Specification](https://www.w3.org/TR/indieauth/)
 - [Flask Logging Documentation](https://flask.palletsprojects.com/en/3.0.x/logging/)
 
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