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>

tests/test_templates.py

@@ -208,19 +208,19 @@ class TestAdminTemplates:
 
     def test_login_template_has_form(self, client):
         """Test login page has form"""
-        response = client.get("/admin/login")
+        response = client.get("/auth/login")
         assert response.status_code == 200
         assert b"<form" in response.data
 
     def test_login_has_me_input(self, client):
         """Test login form has 'me' URL input"""
-        response = client.get("/admin/login")
+        response = client.get("/auth/login")
         assert response.status_code == 200
         assert b'name="me"' in response.data or b'id="me"' in response.data
 
     def test_login_has_submit_button(self, client):
         """Test login form has submit button"""
-        response = client.get("/admin/login")
+        response = client.get("/auth/login")
         assert response.status_code == 200
         assert b'type="submit"' in response.data or b"<button" in response.data