Improve MongoDB Connection, Token Authentication, and Error Handling Consistency

[smog-root]

Currently, there are some improvements that can be made to enhance security, consistency, and maintainability across the server:
Suggested Changes:

MongoDB Connection:

• Update the MongoDB connection string to use an environment variable (process.env.MONGO_URI) instead of a hardcoded string. This allows for easier configuration and enhanced security.

Token Authentication:

• Update JWT_secret to process.env.JWT_SECRET in the authenticateToken middleware for consistency and improved security.

• Modify token handling in authenticateToken to provide more descriptive and standardized responses:

• Return a 401 status code with a message like "Access denied. No token provided" if the token is missing.

• Return a 403 status code with a message like "Invalid token" if token verification fails.

Error Handling Consistency:

• Implement a centralized error handler to standardize error responses. This will eliminate repetitive error handling code and ensure that errors are handled consistently.

Example error handler: 🔽

app.use((err, req, res, next) => {
  res.status(500).json({ error: "An internal server error occurred" });
});

Add gssoc-ext with level and hacktoberfest-accepted labels @mansiruhil13

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