feat(api): enable promotion codes for subscription creation

[mrcfps]

Summary

• Added the allow_promotion_codes flag to the subscription creation process, allowing users to utilize promotion codes during subscription sign-up.
• Ensured compliance with code style rules, including the use of single quotes for string literals.

Tip

Close issue syntax: Fixes #<issue number> or Resolves #<issue number>, see documentation for more details.

Impact Areas

Please check the areas this PR affects:

• Multi-threaded Dialogues
• AI-Powered Capabilities (Web Search, Knowledge Base Search, Question Recommendations)
• Context Memory & References
• Knowledge Base Integration & RAG
• Quotes & Citations
• AI Document Editing & WYSIWYG
• Free-form Canvas Interface
• Other

Screenshots/Videos

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Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Refly Documentation
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods