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

TODO.md

@@ -48,6 +48,23 @@ P2: security: per-origin rate limiting
   times and no longer says the work stops when `downstream_timeout` ends. The
   settings of the WebP, AVIF and JPEG XL encoders, which can hold a slot past
   the default `downstream_timeout` of 60 seconds, are unchanged.
+- 2026-10-15 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 of it kept working for its 30 days. Every login is made
+  with the one signing key, so logging out now ends them all: the session
+  manager keeps a random value, made with `crypto/rand` when the new `logout`
+  table has none and replaced by each logout, saves it there so that it outlasts
+  a restart, puts the value at login in each session, and refuses a session
+  carrying another value. The check for a logout compares no time, so a login
+  after a logout works whatever the host's clock does; a session's 30 days are
+  still checked against the clock. Losing the state directory ends every login
+  session, and no logout can make an ended one work again. Logging out is now
+  `POST /logout` from a form on the generator page, behind the CSRF check the
+  other forms have, so a link or image elsewhere on the same site cannot end the
+  sessions. A logout without a valid login session ends none and only removes
+  the cookie, as anyone can take a CSRF token from the login page. A logout that
+  cannot be saved answers 500. `session.NewManager`, which the tests use, keeps
+  the value in memory only; pixad uses `session.NewManagerWithDatabase`.
 - 2026-10-15 "Running under upaas" in `README.md` tells the operator to set
   `PIXA_CACHE_MAX_BYTES` (closes #251): it listed the variable as optional, but
   under upaas the volume is a directory on the host's filesystem. The default,
@@ -64,22 +81,6 @@ P2: security: per-origin rate limiting
   405, or a CORS preflight. The metrics library takes the label before the
   request is routed, so the metrics middleware asks the router which route
   matches.
-- 2026-10-15 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 of it kept working for its 30 days. Every login is made
-  with the one signing key, so logging out now ends them all: the session
-  manager counts logouts, saves the count in the new `logout` table so that it
-  outlasts a restart, puts the count at login in each session, and refuses a
-  session whose count is not the current one. No time is compared, so a login
-  after a logout works whatever the host's clock does. A restart without a
-  persistent volume for the state directory forgets the count. Logging out is
-  now `POST /logout` from a form on the generator page, behind the CSRF check
-  the other forms have, so a link or image elsewhere on the same site cannot end
-  the sessions. A logout without a valid login session ends none and only
-  removes the cookie, as anyone can take a CSRF token from the login page. A
-  logout that cannot be saved answers 500. `session.NewManager`, which the tests
-  use, keeps the count in memory only; pixad uses
-  `session.NewManagerWithDatabase`.
 - 2026-10-15 expired negative cache rows are removed on a schedule (closes
   #244): the row of a failed fetch, which makes pixa answer 502 for that source
   URL for 5 minutes, was deleted only when the same URL was requested again