```rust
use actix_web::{web, App, HttpServer, HttpResponse, Responder};
use serde::{Deserialize, Serialize};
use std::sync::Mutex;
use std::collections::HashMap;

// Data models
#[derive(Debug, Clone, Serialize, Deserialize)]
struct Welder {
    id: u32,
    name: String,
    articles: Vec<Article>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
struct Article {
    id: u32,
    code: String,
    quantity: u32,
    completed: u32,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
struct Plan {
    id: u32,
    article_code: String,
    total_quantity: u32,
    completed_quantity: u32,
    assignments: HashMap<u32, u32>, // welder_id -> quantity
}

#[derive(Debug, Serialize, Deserialize)]
struct Norm {
    article_code: String,
    time_norm: f32, // hours per unit
}

// App state
struct AppState {
    welders: Mutex<Vec<Welder>>,
    plans: Mutex<Vec<Plan>>,
    norms: Mutex<Vec<Norm>>,
    next_welder_id: Mutex<u32>,
    next_article_id: Mutex<u32>,
    next_plan_id: Mutex<u32>,
}

// REST API handlers
async fn get_welders(data: web::Data<AppState>) -> impl Responder {
    let welders = data.welders.lock().unwrap();
    HttpResponse::Ok().json(welders.clone())
}

async fn add_welder(data: web::Data<AppState>, welder: web::Json<Welder>) -> impl Responder {
    let mut welders = data.welders.lock().unwrap();
    let mut next_id = data.next_welder_id.lock().unwrap();
    let new_welder = Welder {
        id: *next_id,
        name: welder.name.clone(),
        articles: vec![],
    };
    *next_id += 1;
    welders.push(new_welder.clone());
    HttpResponse::Created().json(new_welder)
}

async fn get_plans(data: web::Data<AppState>) -> impl Responder {
    let plans = data.plans.lock().unwrap();
    HttpResponse::Ok().json(plans.clone())
}

async fn add_plan(data: web::Data<AppState>, plan: web::Json<Plan>) -> impl Responder {
    let mut plans = data.plans.lock().unwrap();
    let mut next_id = data.next_plan_id.lock().unwrap();
    let mut new_plan = plan.into_inner();
    new_plan.id = *next_id;
    *next_id += 1;
    plans.push(new_plan.clone());
    HttpResponse::Created().json(new_plan)
}

async fn get_norms(data: web::Data<AppState>) -> impl Responder {
    let norms = data.norms.lock().unwrap();
    HttpResponse::Ok().json(norms.clone())
}

async fn add_norm(data: web::Data<AppState>, norm: web::Json<Norm>) -> impl Responder {
    let mut norms = data.norms.lock().unwrap();
    norms.push(norm.into_inner());
    HttpResponse::Created().json(norms.last().unwrap().clone())
}

async fn assign_article(
    data: web::Data<AppState>,
    path: web::Path<(u32,)>,
    article: web::Json<Article>,
) -> impl Responder {
    let (welder_id,) = path.into_inner();
    let mut welders = data.welders.lock().unwrap();
    let mut next_article_id = data.next_article_id.lock().unwrap();

    if let Some(welder) = welders.iter_mut().find(|w| w.id == welder_id) {
        let new_article = Article {
            id: *next_article_id,
            code: article.code.clone(),
            quantity: article.quantity,
            completed: 0,
        };
        *next_article_id += 1;
        welder.articles.push(new_article.clone());
        HttpResponse::Created().json(new_article)
    } else {
        HttpResponse::NotFound().json("Welder not found")
    }
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    // Initialize app state
    let app_state = web::Data::new(AppState {
        welders: Mutex::new(vec![]),
        plans: Mutex::new(vec![]),
        norms: Mutex::new(vec![]),
        next_welder_id: Mutex::new(1),
        next_article_id: Mutex::new(1),
        next_plan_id: Mutex::new(1),
    });

    // Start HTTP server
    HttpServer::new(move || {
        App::new()
            .app_data(app_state.clone())
            .route("/welders", web::get().to(get_welders))
            .route("/welders", web::post().to(add_welder))
            .route("/welders/{id}/articles", web::post().to(assign_article))
            .route("/plans", web::get().to(get_plans))
            .route("/plans", web::post().to(add_plan))
            .route("/norms", web::get().to(get_norms))
            .route("/norms", web::post().to(add_norm))
    })
    .bind("127.0.0.1:8080")?
    .run()
    .await
}
```