- run_diagnostics composes four module-level probes (_diagnose_database,
_diagnose_dependencies, _diagnose_output_dirs, _diagnose_image_monitor).
- run_maintenance is table-driven: _maintenance_runners maps each
operation name to a callable and _run_maintenance_operations reports
per-operation success or a public error message.
- _read_json answers 400 for malformed or non-object JSON bodies in
settings, duplicate delete, test-link and maintenance instead of 500.
- test_image_link delegates to _link_test_image.
- The four HTML page routes share _serve_html_page.