fix: Change auth blueprint prefix from /admin to /auth (v0.9.2)

The auth routes were registered under /admin/* but the IndieAuth
redirect_uri was configured as /auth/callback, causing 404 errors
when providers redirected back after authentication.

- Change auth blueprint url_prefix from "/admin" to "/auth"
- Update test expectations for new auth route paths
- Add ADR-022 documenting the architectural decision

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

starpunk/routes/auth.py

@@ -27,7 +27,7 @@ from starpunk.auth import (
 )
 
 # Create blueprint
-bp = Blueprint("auth", __name__, url_prefix="/admin")
+bp = Blueprint("auth", __name__, url_prefix="/auth")
 
 
 @bp.route("/login", methods=["GET"])