feat: setup express server with mongodb connection

[adityashirsatrao007]

Issue : #13

🚢 What does this PR do?

This PR establishes the backend foundation for SubSentry, setting up an Express server and connecting it to MongoDB using Mongoose.

🛠️ Changes:

• Switched server to ESM ( ype: module) for modern JS support.
• Implemented a modular database connection in src/config/db.js.
• Refined src/index.js as a clean entry point with standard middleware and health checks.
• Added a dev script for a smoother developer experience using
  odemon.
• Updated .env.example with the necessary environment variables.

✅ Verification:

• Server starts up and attempts connection to MongoDB.
• /api/health returns status and timestamp.
• Modularized DB logic allows for easy testing and scalability.

Verified locally against the project requirements.

[OpenGitBot]

Hey @adityashirsatrao007

Thanks for opening this PR 🚀. Mentor will review your pull request soon and till then, keep contributing and stay calm.

Thanks for contributing in OpenCode'25 ✨✨!

[ShubhamKumarSahu-svg]

@adityashirsatrao007 well done bro, but from next time please add images for reference as well

[OpenGitBot]

Hey @adityashirsatrao007

Your PR has been merged 🥳🥳 and you have earned 30 points.

Thanks for contributing in OpenCode'25✨✨

[Aryan-coder06]

Closing as Week 5 Gmail ingestion milestone is completed and merged into base_files/week5.