Fix database privileges

Fixes psycopg2.errors.InsufficientPrivilege: permission denied for
schema public

nix/modules/fieldpoc.nix

@@ -98,6 +98,7 @@ in {
         CREATE ROLE fieldpoc WITH LOGIN PASSWORD 'fieldpoc' CREATEDB;
         CREATE DATABASE fieldpoc;
         GRANT ALL PRIVILEGES ON DATABASE fieldpoc TO fieldpoc;
+        GRANT USAGE, CREATE ON SCHEMA public TO fieldpoc;
       '';
     };