README: document production env vars, key generation, and restore screenshots

## Summary
The README covers local setup and S3 well but leaves out things a deployer needs:

- Production requires `FLASK_ENV=production` **and** `SCHEMA=` for the schema-prefixed tables and migrations (`app/models/db.py`, `migrations/env.py`); neither is mentioned under "Deploying". `DATABASE_URL` and `SECRET_KEY` aren't listed either.
- `SECRET_KEY`: explain how to generate one instead of copying the example value.
- Spell out the Render build command (`flask db upgrade && flask seed all`) and the Python version (see the dependency issue).
- "Run `pipenv install -r requirements.txt`" ignores `Pipfile` / `Pipfile.lock`; `pipenv install` (or `pipenv sync`) is the intended command.
- `/api/docs` (route list with docstrings) exists but isn't mentioned.
- The screenshots section says the hosted images expired and were removed; commit local PNGs of the landing, login and signup pages under `docs/` and link them.
- Add the future CI badge once the CI issue lands.

Start with the README's Deploying and screenshots sections, then check app/models/db.py, migrations/env.py, Pipfile, and Pipfile.lock for the environment, migration, and dependency details named in the issue. Verify the /api/docs route and the Render build requirements, add the requested deployment guidance and local PNGs under docs/, and ensure all listed links and commands are correct.

Written by the indexing model from the issue text.