Backend to allow changing email during signup

[nc225]

Implements the backend changes to close #9092. Frontend changes are here: #9626. This PR choose option 3 from this comment and lets users change their email at the email verification step. BE changes are necessary here because otherwise, when the user goes to make a new SignupFlow with their correct email address, the desired username is blocked by the old SignupFlow.

I tried to follow the path of our current "ChangeEmailV2" functions, which allow a logged-in user to change the account email address, but for changing signup emails I put the code in auth.py (with the other signup functions) instead of account.py where ChangeEmailV2 lives (with the other logged-in-user functions.) This new functionality also differs from ChangeEmailV2 in that passwords are not involved in changing signup emails, and we don't notify the old_email since it was never verified to begin with. Since we're changing the email in SignupFlow, this invalidates the old email_token and releases the old email to any future Signup

One question for reviewers: do we want to limit the number of times a SignupFlow can change its email address? That's not happening in the current PR but I think if we add another element to SignupFlow in auth.proto we can do that.

Testing

I added a few tests to test_auth.py to check the backend functionality added by this PR. To run the backend tests, I took the following steps:

First I merged the frontend and backend branches into a local branch, reviewers can run:

# Make sure you're in the Couchers repository
cd couchers
git fetch origin
git switch -c review/signup-email-change origin/develop
git merge --no-edit origin/backend/bugfix/signup-email-change-backend
git merge --no-edit origin/web/bugfix/signup-email-change-frontend

Then, a couple steps from the frontend guide: In one terminal, start the local backend:

## terminal 1
cd app
docker run --rm -w /app -v $(pwd):/app registry.gitlab.com/couchers/grpc ./generate_protos.sh
docker compose up

In a second terminal, start the local frontend:

## terminal 2
cd app/web
# use local development config
cp .env.localdev .env.development
# run the frontend (follow Quick Start for prep)
yarn start

To run the backend tests, I opened a third terminal and ran:

## terminal 3
cd app/backend
docker compose exec backend pytest src/tests/test_auth.py

Frontend tests are described in the other PR.

Backend checklist

• Added tests for any new code or added a regression test if fixing a bug
• Run the backend locally and it works
• Added migrations if there are any database changes, rebased onto develop if necessary for linear migration history

Web frontend checklist

• There are no console warnings when running the app
• Added tests where relevant
• Clicked around my changes running locally and it works
• Checked Desktop, Mobile and Tablet screen sizes

For maintainers

• Maintainers can push commits to my branch
• Maintainers can merge this PR for me

[CouchersBot]

Web (Vercel)

View the Vercel web preview for this branch.

Backend

Schema	Schema diff	Sample emails	SQL query log +3/-0 query shapes · 2 tests changed · 8 added, 0 removed

Other

Protos

[aapeliv]

let's fold this into the existing signup rpc