```javascript
require('dotenv').config();
const express = require('express');
const mongoose = require('mongoose');
const cors = require('cors');
const path = require('path');
const bcrypt = require('bcryptjs');
const jwt = require('jsonwebtoken');
const { v4: uuidv4 } = require('uuid');

const app = express();
const PORT = process.env.PORT || 5000;

// Middleware
app.use(cors());
app.use(express.json());
app.use(express.static(path.join(__dirname, 'public')));

// Database Connection
mongoose.connect(process.env.MONGODB_URI || 'mongodb://localhost/tasksphere', {
    useNewUrlParser: true,
    useUnifiedTopology: true,
    useCreateIndex: true
}).then(() => console.log('MongoDB connected'))
  .catch(err => console.log(err));

// Models
const User = require('./models/User');
const Department = require('./models/Department');
const Task = require('./models/Task');

// Authentication Middleware
const authenticate = (req, res, next) => {
    const token = req.header('x-auth-token');
    if (!token) return res.status(401).json({ message: 'No token, authorization denied' });

    try {
        const decoded = jwt.verify(token, process.env.JWT_SECRET);
        req.user = decoded.user;
        next();
    } catch (err) {
        res.status(401).json({ message: 'Token is not valid' });
    }
};

// Routes

// Auth Routes
app.post('/api/auth/register', async (req, res) => {
    const { name, email, password, role } = req.body;

    try {
        let user = await User.findOne({ email });
        if (user) return res.status(400).json({ message: 'User already exists' });

        user = new User({ name, email, password, role });
        const salt = await bcrypt.genSalt(10);
        user.password = await bcrypt.hash(password, salt);

        await user.save();

        const payload = { user: { id: user.id } };
        jwt.sign(payload, process.env.JWT_SECRET, { expiresIn: '5h' }, (err, token) => {
            if (err) throw err;
            res.json({ token });
        });
    } catch (err) {
        console.error(err.message);
        res.status(500).send('Server error');
    }
});

app.post('/api/auth/login', async (req, res) => {
    const { email, password } = req.body;

    try {
        let user = await User.findOne({ email });
        if (!user) return res.status(400).json({ message: 'Invalid credentials' });

        const isMatch = await bcrypt.compare(password, user.password);
        if (!isMatch) return res.status(400).json({ message: 'Invalid credentials' });

        const payload = { user: { id: user.id } };
        jwt.sign(payload, process.env.JWT_SECRET, { expiresIn: '5h' }, (err, token) => {
            if (err) throw err;
            res.json({ token });
        });
    } catch (err) {
        console.error(err.message);
        res.status(500).send('Server error');
    }
});

// Department Routes
app.get('/api/departments', authenticate, async (req, res) => {
    try {
        const departments = await Department.find().populate('manager', 'name');
        res.json(departments);
    } catch (err) {
        console.error(err.message);
        res.status(500).send('Server error');
    }
});

app.post('/api/departments', authenticate, async (req, res) => {
    const { name, description, manager } = req.body;

    try {
        const department = new Department({ name, description, manager });
        await department.save();
        res.json(department);
    } catch (err) {
        console.error(err.message);
        res.status(500).send('Server error');
    }
});

// Task Routes
app.get('/api/tasks', authenticate, async (req, res) => {
    try {
        const tasks = await Task.find()
            .populate('department', 'name')
            .populate('assignedTo', 'name')
            .populate('createdBy', 'name');
        res.json(tasks);
    } catch (err) {
        console.error(err.message);
        res.status(500).send('Server error');
    }
});

app.post('/api/tasks', authenticate, async (req, res) => {
    const { title, description, department, assignedTo, dueDate, priority } = req.body;

    try {
        const task = new Task({
            title,
            description,
            department,
            assignedTo,
            dueDate,
            priority,
            status: 'pending',
            createdBy: req.user.id
        });

        await task.save();
        res.json(task);
    } catch (err) {
        console.error(err.message);
        res.status(500).send('Server error');
    }
});

app.put('/api/tasks/:id', authenticate, async (req, res) => {
    const { status, completedDate } = req.body;

    try {
        let task = await Task.findById(req.params.id);
        if (!task) return res.status(404).json({ message: 'Task not found' });

        task.status = status || task.status;
        if (status === 'completed') {
            task.completedDate = completedDate || new Date();
        }

        await task.save();
        res.json(task);
    } catch (err) {
        console.error(err.message);
        res.status(500).send('Server error');
    }
});

// User Routes
app.get('/api/users', authenticate, async (req, res) => {
    try {
        const users = await User.find().select('-password');
        res.json(users);
    } catch (err) {
        console.error(err.message);
        res.status(500).send('Server error');
    }
});

// Serve static assets in production
if (process.env.NODE_ENV === 'production') {
    app.use(express.static('client/build'));
    app.get('*', (req, res) => {
        res.sendFile(path.resolve(__dirname, 'client', 'build', 'index.html'));
    });
}

app.listen(PORT, () => console.log(`Server running on port ${PORT}`));
```