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/db/migrations/001_schema.sql

@@ -177,11 +177,10 @@ CREATE TABLE IF NOT EXISTS cache_stats (
 );
 INSERT OR IGNORE INTO cache_stats (id) VALUES (1);
 
--- Count of logouts. Each logout adds one; a login session carries the count
--- at its login and is refused once the count has changed, so logging out ends
--- every login session.
+-- A random value that a login session carries from its login. pixa makes it
+-- when the table has no row, and each logout replaces it; a login session
+-- carrying another value is refused, so logging out ends every login session.
 CREATE TABLE IF NOT EXISTS logout (
     id INTEGER PRIMARY KEY CHECK (id = 1),
-    logout_count INTEGER NOT NULL DEFAULT 0
+    logout_value TEXT NOT NULL
 );
-INSERT OR IGNORE INTO logout (id) VALUES (1);