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

TODO.md

@@ -47,6 +47,17 @@ P2: security: per-origin rate limiting
   answered 500. Once the limit is used up, the fetcher now reads one more byte:
   if the body has ended it is accepted, and if there is more it is refused with
   `ErrResponseTooLarge`, as before.
+- 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 `/logout` now ends them all: the session manager
+  keeps the time of the latest logout, saves it in the new `logout` table so
+  that it outlasts a restart, and refuses a session made before it. A `/logout`
+  without a valid login session, such as one another site makes a browser send
+  without the `SameSite=Strict` cookie, ends none and only removes the cookie. A
+  logout that cannot be saved answers 500. `session.NewManager`, which the tests
+  use, keeps logouts in memory only; pixad uses
+  `session.NewManagerWithDatabase`.
 - 2026-10-13 `fit=outside` no longer makes an output above 8192 on a side
   (closes #257): it scales the image to cover the requested size, and the side
   scaled past that size was held to no maximum, so `1x8192?fit=outside` of a