feat: Refactor configuration handling into centralized validation layer

[vraj826]

Description

Implemented a centralized configuration layer and startup validation flow to provide a single source of truth for application settings while preserving existing behavior.

Related Issue

Closes #177

Type of Change

• Refactor

Changes Made

• Centralized environment variable loading and normalization through the configuration module

• Wired configuration consumption through:

  • server startup
  • cache utilities
  • GitHub services
  • analytics services
  • profile query defaults

• Added configuration validation and startup diagnostics

• Added focused tests for configuration behavior and defaults

• Updated missing-username test to use DEFAULT_USERNAME

• Updated .env.example

• Updated README configuration documentation

Testing

• Ran npm test — all tests pass
• Tested manually (if applicable)

Verification performed:

• npm.cmd run lint → passing
• npm.cmd run test:unit → passing (13 suites, 177 tests)

Screenshots (if applicable)

N/A

Checklist

• My code follows the project's code style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings or errors
• I have updated the documentation if necessary
• My changes are scoped to a single issue

[vercel]

@vraj826 is attempting to deploy a commit to the Dot_NotSam's projects Team on Vercel.

A member of the Team first needs to authorize it.

[vraj826]

@SamXop123 Can you review the changes!

[SamXop123]

lgtm. thank you for the contribution

[vercel]

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Project	Deployment	Actions	Updated (UTC)
samdev-pulse	Ready	Preview, Comment	Jun 14, 2026 5:40pm

[github-actions]

🎉 Congratulations @vraj826! 🎉

Your pull request has been successfully merged into samdev-pulse! Thank you so much for your valuable contribution and effort. Every single improvement helps make samdev-pulse a better profile experience for everyone! 🙌

🚀 What's Next?

• Keep the momentum going! Feel free to browse through our open Issues and grab another one that interests you!
• Support the Project: If you enjoyed contributing to samdev-pulse, please take a second to star our repository! It helps increase the project's visibility and supports our community. ⭐

💬 Stay Connected:
Have any questions, want feedback, or just want to discuss further contributions? Feel free to reach out directly to the maintainer on Discord: dot_notsam

Thank you again, and keep up the amazing work! 💻✨