feat: add logout button to admin interface (Story 1.4)

Add logout button to admin dashboard and test to verify its presence.
This completes the missing acceptance criterion for Story 1.4:
"Logout option available from admin interface"

Changes:
- Add logout form with CSRF protection to dashboard header
- Add integration test to verify logout button is present
- All tests pass (24/24)

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

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

tests/integration/test_admin_login.py

@@ -354,3 +354,29 @@ class TestAdminLogin:
         )
         assert response.status_code == 200
         assert b"required" in response.data.lower()
+
+    def test_logout_option_available_in_admin_interface(self, client, db, admin):  # noqa: ARG002
+        """Test that logout option is available from admin interface.
+
+        Acceptance Criteria (Story 1.4):
+        - Logout option available from admin interface
+        """
+        # Login first
+        client.post(
+            "/admin/login",
+            data={
+                "email": "admin@example.com",
+                "password": "testpassword123",
+            },
+            follow_redirects=False,
+        )
+
+        # Access admin dashboard
+        response = client.get("/admin/dashboard", follow_redirects=False)
+        assert response.status_code == 200
+
+        # Verify logout button/link is present
+        # Check for logout form posting to /admin/logout
+        assert b"/admin/logout" in response.data
+        # Check for logout text in button or link
+        assert b"logout" in response.data.lower() or b"log out" in response.data.lower()