feat: design and implement core subscription model

[adityashirsatrao007]

Issue : #14

🚢 What does this PR do?

This PR implements the core data model for tracking subscriptions in SubSentry.

🛠️ Changes:

• Designed a comprehensive Subscription Mongoose schema with support for various billing cycles, statuses, and data sources.
• Ensured strong data validation for price, name, and billing cycles.
• Implemented user ownership mapping via userClerkId, aligned with the frontend Clerk integration.
• Added indexing on userClerkId and status to optimize common query patterns.
• Included automatic timestamps for auditing.

✅ Verification:

• Model schema is clean, extensible, and correctly follows the ESM architecture.
• Validated that all required fields are captured to support future automation.

Verified against the project's backend 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 ✨✨!

[OpenGitBot]

Hey @adityashirsatrao007

Your PR has been merged 🥳🥳 and you have earned 10 points.

Thanks for contributing in OpenCode'25✨✨

[Aryan-coder06]

Closing as requested by maintainer.