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.go

@@ -100,6 +100,35 @@ type Session struct {
 	now func() time.Time
 }
 
+// cookieOptions returns the cookie attributes used for every session
+// cookie. MaxAge is deliberately left at its zero value: for a store
+// it is set through CookieStore.MaxAge (see newStore), and for a
+// single session it is copied from the store's options.
+func cookieOptions(secure bool) *sessions.Options {
+	return &sessions.Options{
+		Path:     "/",
+		HttpOnly: true,
+		Secure:   secure,
+		SameSite: http.SameSiteLaxMode,
+	}
+}
+
+// newStore builds the session cookie store.
+//
+// The absolute cap MUST be applied with store.MaxAge and not by
+// assigning store.Options.MaxAge. NewCookieStore gives the underlying
+// securecookie codecs a 30-day max age of their own, and assigning
+// Options never touches Codecs -- so a store configured that way still
+// decodes a 30-day-old cookie, leaving the cookie attribute and the
+// codec disagreeing about the same policy. store.MaxAge sets both.
+func newStore(key []byte, secure bool) *sessions.CookieStore {
+	store := sessions.NewCookieStore(key)
+	store.Options = cookieOptions(secure)
+	store.MaxAge(secondsPerDay * sessionMaxAgeDays)
+
+	return store
+}
+
 // New creates a new session manager. The cookie store is
 // initialized during the fx OnStart phase after the database is
 // connected, using a session key that is auto-generated and stored
@@ -142,19 +171,8 @@ func New(
 				)
 			}
 
-			store := sessions.NewCookieStore(keyBytes)
-
-			// Configure cookie options for security
-			store.Options = &sessions.Options{
-				Path:     "/",
-				MaxAge:   secondsPerDay * sessionMaxAgeDays,
-				HttpOnly: true,
-				Secure:   !params.Config.IsDev(),
-				SameSite: http.SameSiteLaxMode,
-			}
-
 			s.key = keyBytes
-			s.store = store
+			s.store = newStore(keyBytes, !params.Config.IsDev())
 			s.log.Info("session manager initialized")
 
 			return nil
@@ -350,13 +368,8 @@ func (s *Session) Regenerate(
 	// Apply the standard session options (the destroyed old
 	// session had MaxAge = -1, which store.New might inherit
 	// from the cookie).
-	newSess.Options = &sessions.Options{
-		Path:     "/",
-		MaxAge:   secondsPerDay * sessionMaxAgeDays,
-		HttpOnly: true,
-		Secure:   !s.config.IsDev(),
-		SameSite: http.SameSiteLaxMode,
-	}
+	newSess.Options = cookieOptions(!s.config.IsDev())
+	newSess.Options.MaxAge = secondsPerDay * sessionMaxAgeDays
 
 	return newSess, nil
 }