Feat: budget management

[MRawhani]

Summary

Budget management service for AI cost enforcement, log status management, and concurrency control for chat requests.

New User Features

• None (internal service module)

New Dev Features

• New budget.js service module handling:
  • AI budget enforcement with two caps checked per request:
    1. Model global cap (ai_model.costLimit) : sum across all users
    2. Per-user share cap (ai_model_share.costLimit) - individual user spend
  • Each cap has its own resetAt
  • One in-flight request per (userId, studySessionId) - second request is rejected
  • Usage computed from ai_log on demand
  • In-flight ai_log rows have NULL cost and contribute $0 to sums
• New migrations for additional columns in ai_model and ai_model_share tables
• Updated schema definitions for both models

Improvements

• None

Bug Fixes

• None

Known Limitations

• chat.js and share.js not yet updated to call the new budget functions (beginRequest, cancel, etc.)
• Bulk delete (wipe) of all shares when creating a new share not yet removed
• Reset and cancellation functionality not yet exposed in backend
• Study creation does not yet support creating a share with a cost limit

Future Steps

• Adjust current service files (e.g. chat.js, share.js) to call the new budget functions such as beginRequest, cancel, etc.
• Remove the wipe (bulk delete) of all shares when creating a new share.
• Ensure params are fully prepared and parsed before being passed to budget.js functions ; preprocessing must happen within the calling service file; budget.js will save them as-is (see params doc in budget file).
• Treat budget.js as the sole responsible module for AI budget enforcement, AI log status management, and concurrency for chat requests.
• Add necessary new functionalities: e.g. add the cost limit for model in frontend and backend, allow study creation to include a share with a cost limit, add reset and cancellation functionality, and call budget service where needed.

[akash9676]

Name of the file is wrong.