The database step drops the session key from .env #667

Problem

updateDB (POST /wizard_db) rewrites .env with only the database settings. The SESSION_KEY line that startup wrote a moment earlier is lost. The running process keeps its key in memory, so nothing shows until the first restart: goblog then finds no key, generates a new one, and every existing session cookie stops verifying. The admin who just finished the install is signed out by the first restart.

Fix

Keep the existing lines when writing the database settings (rewrite only the database keys), or write the session key back with them.

Predates #664; found while testing it.