fix(auth): stateless session cookie — stop logging users out on deploy (ADR-029)

Pilot feedback: lots of re-logging-in on iPhone. Three causes: sessions lived in
an in-memory map (wiped on every pod restart/deploy), a 1h TTL (idle >1h forced
re-login on a check-back-tomorrow reader), and a session cookie with no Max-Age
(dropped on Safari close). Each re-login is the full IdP redirect dance.

Make sessions stateless: identity + absolute expiry live inside the existing
HMAC-signed cookie (no server table), TTL 1h → 30 days sliding, cookie now
persistent (Max-Age). Survives restarts (test: a cookie from one instance is
accepted by a fresh instance with the same secret), browser-close, and idle.
Trade: no server-side revocation — logout clears the cookie client-side; rotating
tapir-session-secret is the global logout lever. Accepted for the Stage-0 reader.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/web/oidc/oidc_test.go

@@ -306,13 +306,42 @@ func TestLogoutClearsSession(t *testing.T) {
 	require.Equal(t, http.StatusFound, rec.Code)
 	require.Equal(t, "/welcome", rec.Header().Get("Location"), "logout lands on the public page")
 	cleared := sessionCookie(t, rec.Result())
-	require.Less(t, cleared.MaxAge, 0, "logout expires the cookie")
+	require.Less(t, cleared.MaxAge, 0, "logout expires the cookie so the browser drops it")
+	require.Empty(t, cleared.Value, "logout blanks the cookie value")
 
-	// The server-side session is gone: the original cookie no longer resolves.
+	// Sessions are stateless (ADR-029): logout clears the cookie client-side, so a
+	// request carrying the cleared (empty) cookie is unauthenticated. The original
+	// signed cookie remains technically valid until its expiry — the accepted
+	// trade for no server-side store; the browser no longer holds it.
 	check := httptest.NewRequest(http.MethodGet, "/", nil)
-	check.AddCookie(cookie)
+	check.AddCookie(cleared)
 	_, ok := auth.CurrentUser(check)
-	require.False(t, ok)
+	require.False(t, ok, "the cleared cookie does not authenticate")
+}
+
+// TestSessionSurvivesRestart is the core of ADR-029: a cookie issued by one
+// process is accepted by a FRESH instance with the same session secret — so a
+// deploy/pod-restart no longer logs users out (the old in-memory store did).
+func TestSessionSurvivesRestart(t *testing.T) {
+	f := newFakeIssuer(t)
+	auth1 := newAuth(t, f)
+	cookie := authenticate(t, auth1, f)
+
+	auth2 := newAuth(t, f) // simulate a redeploy: new process, same SessionSecret
+	req := httptest.NewRequest(http.MethodGet, "/", nil)
+	req.AddCookie(cookie)
+	user, ok := auth2.CurrentUser(req)
+	require.True(t, ok, "a session must survive a restart (stateless signed cookie)")
+	require.Equal(t, testSubject, user.Subject)
+}
+
+// TestSessionCookieIsPersistent: the cookie carries a positive Max-Age so it
+// survives the browser/app being closed (a session cookie was dropped on iOS).
+func TestSessionCookieIsPersistent(t *testing.T) {
+	f := newFakeIssuer(t)
+	auth := newAuth(t, f)
+	cookie := authenticate(t, auth, f)
+	require.Greater(t, cookie.MaxAge, 0, "session cookie must be persistent (Max-Age set)")
 }
 
 func TestExpiredSessionRejected(t *testing.T) {