Let a browser with cookies from an earlier database log in (closes #359)

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

internal/session/session.go

@@ -224,10 +224,22 @@ func New(
 }
 
 // Get retrieves a session for the request.
+//
+// A session cookie that does not decode -- one signed with an earlier
+// session key, say, because the database was made anew -- is treated
+// as absent: the caller gets a new, empty session and no error, and
+// the next save replaces the cookie.
 func (s *Session) Get(
 	r *http.Request,
 ) (*sessions.Session, error) {
-	return s.store.Get(r, SessionName)
+	sess, err := s.store.Get(r, SessionName)
+	if sess == nil {
+		return nil, err
+	}
+
+	// For a cookie that does not decode, gorilla/sessions returns a
+	// new, empty session alongside the error that is dropped here.
+	return sess, nil
 }
 
 // GetKey returns the raw 32-byte authentication key used for