feat: implemented user login endpoint and JWT generation

[ADARSHsri2004]

Description

Implemented the user login endpoint (POST /api/auth/login) for authentication.
This endpoint validates user credentials, compares hashed passwords using bcryptjs, and returns a JWT containing the user’s ID and role upon successful login.

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Semver Changes

• Minor (new features, no breaking changes)

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Issues

Closes: #12

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Acceptance Criteria

• Create the controller, service, and route for login.
• Find the user by email in the database.
• Compare the provided password with the stored hash using bcryptjs.
• If credentials are valid, generate a JWT containing the user's ID and role.
• Return the generated JWT to the client.

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Checklist

• I have read the Contributing Guidelines
• My code follows the project’s coding style and structure.
• I have tested all new functionality manually.
• I have added all required labels:
  • Type: Feature
  • Semver: Minor
  • PR:Accept