📌 Add authentication to demo

[TheKinrar]

While demo already has an authentication UI, it's completely client-side and therefore does not provide any authentication for server requests.

What needs to be done:

• Add API endpoint in the demo backend to exchange an email auth token for a blindnet token
• Use this endpoint in PRCI to authenticate users, then use the returned token to authenticate requests to PCE

We also need to authenticate DCI requests, but these are app-scoped, so it's a bit different, we don't need email verification. Instead, we need to:

• add an API endpoint (probably password-authenticated for demo purposes?) to get an app-scope blindnet token
• and then use this token for DCI requests to PCE

[milstan]

Same needs to be done on the dogfood Privacy Portal and Talent Portal