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

sessions.NewCookieStore gives its securecookie codecs a 30-day max
age, and assigning store.Options never touches Codecs. The store
therefore decoded a cookie up to 30 days old while the cookie
attribute and the server-side expiry check both said 7 days, so a
refactor of that check -- or a second decode path that skips
IsAuthenticated -- would silently reopen a 30-day window.

Build the store through store.MaxAge, which sets Options.MaxAge and
propagates to every codec. The store is now built by newStore, the
one place a store is constructed; Regenerate shares its cookie
attributes via cookieOptions.

Tests:

- Two codec tests decode a re-stamped cookie an hour inside and an
  hour outside the cap. They only exercise Session.Get, so they pin
  the codec: reverting the fix fails the rejection test whether or
  not the server-side expiry check is present.
- The lazy-refresh bound is pinned two-sidedly, so the documented
  "expires up to 10% early, never late" guarantee now fails the
  suite if idleRefreshDivisor drifts in either direction.

Also scopes the RequireAuth save error to a distinct saveErr
variable instead of reusing the outer err. It is a plain assignment
rather than an inline "if saveErr := ...; saveErr != nil", because
the repo's noinlineerr linter rejects that form. Behaviour is
unchanged; this is scoping hygiene, not a bug fix. Two README claims
are corrected as well: the idle timeout is disabled by any
non-positive value, not only 0, and deploying the two-clock expiry
logs existing sessions out once because they carry no timestamps.

#108

internal/session/session_test.go

@@ -39,6 +39,19 @@ func (c *fakeClock) Advance(d time.Duration) {
 	c.t = c.t.Add(d)
 }
 
+// testKey returns the fixed session key the tests sign with. The
+// codec tests re-sign cookies with it, so it must be the same key the
+// store was built from.
+func testKey() []byte {
+	key := make([]byte, testKeySize)
+
+	for i := range key {
+		key[i] = byte(i + 42)
+	}
+
+	return key
+}
+
 // testSession creates a Session with a real cookie store and the
 // real clock.
 func testSession(t *testing.T) *session.Session {
@@ -59,20 +72,8 @@ func testSessionWithClock(
 ) (*session.Session, *fakeClock) {
 	t.Helper()
 
-	key := make([]byte, testKeySize)
-
-	for i := range key {
-		key[i] = byte(i + 42)
-	}
-
-	store := sessions.NewCookieStore(key)
-	store.Options = &sessions.Options{
-		Path:     "/",
-		MaxAge:   86400 * 7,
-		HttpOnly: true,
-		Secure:   false,
-		SameSite: http.SameSiteLaxMode,
-	}
+	key := testKey()
+	store := session.NewStore(key, false)
 
 	cfg := &config.Config{
 		Environment:        config.EnvironmentDev,
@@ -645,6 +646,34 @@ func TestTouch_LazyBelowRefreshThreshold(t *testing.T) {
 	)
 }
 
+func TestTouch_RefreshThresholdIsOneTenthOfIdleWindow(t *testing.T) {
+	t.Parallel()
+
+	// testRefreshDivisor restates the documented bound independently
+	// of the implementation constant: the idle timestamp is rewritten
+	// once it is a tenth of the idle window old, which is what makes
+	// "expires up to 10% early, never late" true. Both assertions are
+	// needed to pin it -- a larger divisor fails the first, a smaller
+	// one fails the second.
+	const testRefreshDivisor = 10
+
+	threshold := testIdleTimeout / testRefreshDivisor
+
+	s, sess, clock := authenticatedSession(t, testIdleTimeout)
+
+	clock.Advance(threshold - time.Second)
+	assert.False(
+		t, s.Touch(sess),
+		"Touch must not rewrite the session below a tenth of the window",
+	)
+
+	clock.Advance(time.Second)
+	assert.True(
+		t, s.Touch(sess),
+		"Touch must rewrite the session at a tenth of the window",
+	)
+}
+
 func TestTouch_UnauthenticatedSessionIsNotRefreshed(t *testing.T) {
 	t.Parallel()