Backend developer for an AI reading app: database, accounts, shared book storage

I'm a solo founder of a web app that helps parents coach their child's reading. A beta with real families is finished. The main complaint was slow loading of books and questions. The app has no backend yet, and I'm hiring an experienced backend developer to build one. What the backend should do: 1. Database and accounts: accounts, children, books, and sessions. 2. Shared book storage: when one family loads a book, the work is saved so other families get it much faster. 3. Move AI requests onto a server, with per-user limits, so credentials stay off users' devices. This needs attention at the start, first or alongside the first piece of work. 4. Several children per account, with a way to switch between them. 5. Basic visibility into usage and errors, and backups. 6. A way to move existing beta testers' data over so nobody loses what they have. I have not chosen any tools. Suggestions with reasons are welcome. A native mobile app is not part of this job. It is a later, separate phase. Child and family data stays private. Only general book-level content is meant to be shared. How hiring will work: applying is unpaid, and I'm not asking for a free audit or detailed review. I will choose from the proposals, then agree on scope and price with the person I pick. After that, I will walk you through the app as part of the paid work. I will share the app and code only after a signed NDA and IP assignment. Please do not ask for them before then. If you think a short paid code review should come before a fixed price, say so in your proposal and price it separately. That is optional. To apply, please send: - Your approach to this backend, in your own words, based on what is described here. - Two examples of speeding up or lowering the cost of an AI-powered app: what you measured and what changed. - Your experience with accounts, databases, and migrating existing user data. - Your estimated price and timeline for the build, and what could change them once you see the app. - When you could start.