Contacts now carry a user_id FK to users. When a logged-in user
submits a contact (single or CSV upload), the server injects their
user_id and the email from their account into the submission params,
and the visible email input is replaced with a hidden input holding
that value — logged-in users don't have to retype their address.
The submission changeset now requires at least one of user_id or
submitter_email (rather than hard-requiring email), and email format
is only validated when one is actually present.
Reworded the submit-page blurb from "10 GHz to 241 GHz" to "902 MHz
and up" so it matches the actual band coverage of the project.
