I recommend Ruby/Rails. I write the text in question answer form:
Quick prototyping
Yes you can use the prototyping in Rails or not. And this can be realized by adding any of testing framework like Cucumber or RSpec for example. Writing the test you can work on the application prototype without a change real code in production, or even development environment.
Easy to get to production level for backend
Yes it is easy, you are able to use the capistrano gem with its modules, if required, to deploy application to production within a day.
Maintainable and buildable by one person alone
Since the Rails gets on itself the main code part of a data processing, you are able easily to generate the required code with build-in Rails or external gem generators, and to merely maintain the your code, if you would cover it with a test of selected test framework.
Enforces conformity, so others can easily read and contribute to the code, making successfully open sourcing possible
As far as I said you can barely control a contribution to your project with using the test covering frameworks, to keep conformity the project.
Works well with React
You can use React along with any JS NPM module easily using webpacker gem.
Has a mature and fairly stable ecosystem for the basics (auth, storage, db, image handling, backups, change history, etc.
Of course the Rails framework is supported by many three-side gems, which can be found at rubygems, that support the authentication (devise), storage feature (fog), db (pg/mysql/sqlite3), image handling (rmagick), backups (backup), change history (history)
Has an ecosystem that will still be alive in 10 years, responding to changes.
Ruby/Rails with the RubyGem ecosystem is actively envolving and not plan to be collapsed and abandned with of development both Ruby language and Rails framework.
So, you can freely and fearlessly start using the Rails framework as server side base for your project.