Logging out ends every login session (closes #256)

A login session was a cookie with nothing kept on the server, so /logout
removed it from one browser and a copy kept working for 30 days. Every
login is made with the one signing key, so /logout now ends them all:
the session manager keeps the time of the latest logout, saves it in the
new logout table so it outlasts a restart, and refuses a session made
before it. A /logout without a valid session ends none and only removes
the cookie, so another site cannot log everyone out. A logout that
cannot be saved answers 500. session.NewManager, which existing tests
use, keeps logouts in memory only; pixad uses NewManagerWithDatabase.

Model: opus-5-5

internal/db/migrations/001_schema.sql

@@ -176,3 +176,10 @@ CREATE TABLE IF NOT EXISTS cache_stats (
     last_updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
 );
 INSERT OR IGNORE INTO cache_stats (id) VALUES (1);
+
+-- Time of the latest logout. A login session made before it is refused,
+-- so logging out ends every login session. No row: nobody has logged out.
+CREATE TABLE IF NOT EXISTS logout (
+    id INTEGER PRIMARY KEY CHECK (id = 1),
+    logged_out_at DATETIME NOT NULL
+);