- Add a shared common error response schema and unknown error code for business-level fallbacks
- Return business error payloads for router NoRoute and NoMethod fallbacks
- Align the existing event list fallback response with the shared error structure
- Introduce an explicit unimplemented API error type instead of relying on formatted strings
- Make /api batch dispatch return per-item status 600 error entries for unimplemented modules and actions without failing the whole batch
Signed-off-by: Sean Du <do4suki@gmail.com>