Fix stack trace exposure alert

[yusufm]

Summary

• stop returning exception-derived text from default API error responses
• keep fixed public error messages for known 4xx categories and explicit caller-provided messages
• replace macOS-incompatible entrypoint sed usage with a portable config rewrite

Tests

• python3 -m py_compile web/app.py
• pytest tests/test_api_error_sanitization.py tests/test_web_route_smoke.py
• pytest tests/test_entrypoint_bootstrap.py::test_entrypoint_bootstrap_is_safe_under_parallel_first_run tests/test_api_error_sanitization.py -q
• pytest