Replace the count of logouts with a random value (part of #256)

The session manager now keeps a random value from crypto/rand in the
logout table, made when the table has none and replaced by each logout.
A session carries the value current at its login and is refused when it
differs. A lost state directory now ends every login session, and no
logout can make an ended one work again, as a count starting from 0
could. README.md and the Manager doc comment say the check for a logout
compares no time, while a session's expiry still compares the clock.

Model: opus-5-5

internal/handlers/handlers.go

@@ -164,8 +164,8 @@ func (s *Handlers) initImageService() error {
 
 	// Initialize session manager (signing key is validated at config load
 	// time). Session cookies are always Secure/HttpOnly/SameSite=Strict.
-	// The count of logouts is saved in the database, so it outlasts a
-	// restart.
+	// The random value that each logout replaces is saved in the database,
+	// so it outlasts a restart.
 	sessMgr, err := session.NewManagerWithDatabase(
 		context.Background(), s.config.SigningKey, s.db.DB())
 	if err != nil {