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_routes_dev_auth.py

@@ -255,7 +255,7 @@ class TestDevModeWarnings:
     def test_dev_login_page_shows_link(self, dev_app):
         """Test login page shows dev login link when DEV_MODE enabled"""
         client = dev_app.test_client()
-        response = client.get("/admin/login")
+        response = client.get("/auth/login")
         assert response.status_code == 200
 
         # Should have link to dev login
@@ -264,7 +264,7 @@ class TestDevModeWarnings:
     def test_production_login_no_dev_link(self, prod_app):
         """Test login page doesn't show dev link in production"""
         client = prod_app.test_client()
-        response = client.get("/admin/login")
+        response = client.get("/auth/login")
         assert response.status_code == 200
 
         # Should NOT have dev login link
@@ -335,7 +335,7 @@ class TestIntegrationFlow:
         # Step 1: Access admin without auth (should redirect to login)
         response = client.get("/admin/", follow_redirects=False)
         assert response.status_code == 302
-        assert "/admin/login" in response.location
+        assert "/auth/login" in response.location
 
         # Step 2: Use dev login
         response = client.get("/dev/login", follow_redirects=True)
@@ -358,7 +358,7 @@ class TestIntegrationFlow:
         assert response.status_code == 200
 
         # Step 5: Logout
-        response = client.post("/admin/logout", follow_redirects=True)
+        response = client.post("/auth/logout", follow_redirects=True)
         assert response.status_code == 200
 
         # Step 6: Verify can't access admin anymore