A new database brings a new session key. A browser still holding the
old session cookie got a 500 on a correct login: Session.Get returned
the cookie's decode error and the login handler answered it with a
500. Get now treats a cookie that does not decode as absent, and
logging in replaces it. gorilla/csrf already did the same for the
CSRF cookie.
A start that creates webhooker.db now logs "created a new, empty
database" at WARN with its path, shortly before the first-boot banner,
so an unexpectedly empty DATA_DIR is noticed.
The codec tests now decode through the store, since Get no longer
reports the codec's reason.
Model: opus-5-5