Add inactivity-based session timeout (closes #66) (#105)

Sessions now carry a server-enforced idle deadline (SESSION_IDLE_TIMEOUT,
default 24h) alongside the 7-day absolute cap, refreshed on authenticated
activity. Activity never extends the absolute cap.

README.md

@@ -93,6 +93,27 @@ TTY detection, and security headers are always applied.
 | `METRICS_USERNAME`      | Basic auth username for `/metrics`  | `""`     |
 | `METRICS_PASSWORD`      | Basic auth password for `/metrics`  | `""`     |
 | `SENTRY_DSN`            | Sentry error reporting DSN          | `""`     |
+| `SESSION_IDLE_TIMEOUT`  | Idle session timeout (Go duration)  | `24h`    |
+
+Sessions are bounded by two independent clocks, and end at whichever
+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.
+- **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.
+
+Only requests that authenticate with the session count as activity, so
+an unauthenticated request carrying the cookie cannot keep a session
+alive. The idle timestamp is rewritten at most once per tenth of the
+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.
 
 #### Invalid values abort startup