Enhance BlueSky integration to use token-based authentication

[coderabbitai]

Background

The current implementation in packages/studiocms_socialposter/src/routes/postToBlueSky.ts logs in for every post request to BlueSky. This approach, while functional, creates unnecessary network calls and authentication overhead.

Enhancement Proposal

Refactor the BlueSky integration to use token-based authentication (JWT) which is supported by the BlueSky API:

• Implement a mechanism to perform a single initial login that returns access and refresh tokens
• Store and reuse the access token for subsequent requests
• Add logic to refresh the token when it expires
• Use the token in request headers for authenticated operations

Benefits

• Reduced network calls and better performance
• Alignment with BlueSky API best practices
• More resilient authentication flow

References

• PR discussion: Add new social media poster plugin #6 (comment)

This issue was created as a TODO item based on a review comment in PR #6.