#84 FTX-84: 500 instead of 422 envelope on unparseable request body

EARS SPEC: - If a request body cannot be parsed or validated (including non-JSON or non-UTF-8 bytes input), then the API shall return the 422 validation_error envelope (UB-API-6), never a 500. - The deploy smoke script shall send Content-Type: application/json on its JSON requests. Found during FTX-81 prod smoke: RequestValidationError.errors() embeds the raw bytes input; error_response -> JSONResponse raises TypeError -> generic 500. Repro: curl -X POST /v1/tenants with a JSON string body but no Content-Type header (form-encoded default).