Logging out ends every login session (closes #256)

A login session was a cookie with nothing kept on the server, so
logging out removed it from one browser and a copy kept working for 30
days. The session manager now keeps a random value in the database,
made when it has none and replaced by each logout; a session carries
the value current at its login and is refused once it has changed. The
logout check compares no time, so the host's clock cannot block a
login, and losing the state directory ends every login session.
Logging out is now a POST with the CSRF token, sent by a form on the
generator page.

Model: opus-5-5

internal/db/migrations/001_schema.sql

@@ -176,3 +176,11 @@ CREATE TABLE IF NOT EXISTS cache_stats (
     last_updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
 );
 INSERT OR IGNORE INTO cache_stats (id) VALUES (1);
+
+-- 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_value TEXT NOT NULL
+);