Add GitHub OAuth integration for repo indexing via existing pipeline

[Nirvana77]

Summary

This PR adds GitHub OAuth App support for indexing repositories using the existing pipeline (github_to_entries.py). It allows users to authenticate via GitHub and select repositories for indexing using access tokens.

This is primarily a self-contained feature and does not affect other parts of the codebase outside the GitHub integration.

---

What’s Included

• New API endpoints for GitHub OAuth authentication and repo selection (api_github.py)
• .env support for:
  • GITHUB_CLIENT_ID
  • GITHUB_CLIENT_SECRET
  • GITHUB_REDIRECT_URI
• Integration with the existing GitHub content processor (github_to_entries.py)
• GitHub helper logic to handle OAuth exchange and repo listing

---

Notes

• Repo data is only updated when the user reselects the repository from the /settings/content/github UI.
• A background job for auto-refresh is being experimented with but is not included in this PR.

---

Testing

To test this PR, you'll need to create a GitHub OAuth App and add the following environment variables:

GITHUB_CLIENT_ID=your-client-id
GITHUB_CLIENT_SECRET=your-client-secret
GITHUB_REDIRECT_URI=http://localhost:42110/api/github/callback

Let me know if you'd like me to include setup instructions or a sample .env file in the repo/docs.

---

Related

• Based on prior work by @sabaimran in github_to_entries.py
• Mentioned in Issue #1037 and Issue #1003
• 

[Nirvana77]

Hi @debanjum, the PR is ready for review whenever you have time. Let me know if you'd like any changes or additional tests.

PR link: #1175