Add SESSION_COOKIE_DOMAIN so /admin SSR sees the session cookie

Production splits web (descrybe.io) and API (api.descrybe.io). The session
cookie was host-only for api.descrybe.io, so the browser never sent it to
the web host. The /admin SvelteKit SSR gate (fetchMeStaff in
+layout.server.ts) forwards the incoming cookie header to /api/auth/me —
with no cookie to forward it always got 401 and bounced every successful
login back to /login?next=/admin (login POST 200, /me 200 from the
browser, /me 401 from the web server).

New SESSION_COOKIE_DOMAIN env (default empty = host-only, local dev
unchanged) sets the session cookie Domain attribute; set it to the parent
domain (descrybe.io) in production so both hosts receive the cookie.
Leading dot is normalized away. CSRF needs no change — it already seeds
cross-origin via the X-CSRF-Token response header.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/api/cmd/api/main.go

@@ -56,7 +56,7 @@ func main() {
 	}
 	defer pool.Close()
 
-	sessions := auth.NewSessionManager(pool, cfg.SessionCookieName, cfg.CookieSecure(), cfg.SessionIdleHours)
+	sessions := auth.NewSessionManager(pool, cfg.SessionCookieName, cfg.SessionCookieDomain, cfg.CookieSecure(), cfg.SessionIdleHours)
 	srv := httpapi.NewServer(cfg, pool, sessions)
 
 	runCtx, runCancel := context.WithCancel(context.Background())