Align session codec max-age with the 7-day cap (closes #108)

README.md

@@ -168,9 +168,10 @@ one runs out first:
 - **Idle expiry** (`SESSION_IDLE_TIMEOUT`, default `24h`) is a sliding
   window. Every authenticated request pushes it forward, so a session
   in continuous use never hits it, while an abandoned one expires a day
-  after its last use. Set it to `0` to disable idle expiry entirely;
-  the absolute cap below still applies. A set-but-unparseable value
-  aborts startup rather than silently falling back to the default.
+  after its last use. Any non-positive value (`0`, or a negative
+  duration such as `-1s`) disables idle expiry entirely; the absolute
+  cap below still applies. A set-but-unparseable value aborts startup
+  rather than silently falling back to the default.
 - **Absolute expiry** is a fixed 7 days from login. Activity does
   **not** extend it: after a week, every session ends and the user
   authenticates again.
@@ -182,6 +183,11 @@ idle window rather than on every request, which means a session may
 expire up to 10% early relative to the user's true last request, but
 never late.
 
+Both clocks are anchored by timestamps stored in the session cookie.
+Sessions issued before this feature existed carry neither, so they are
+treated as expired: upgrading to a build that has it logs every
+existing session out once, and those users sign in again.
+
 #### Invalid values abort startup
 
 The defaults above apply **only** to variables that are unset (or set