feat: auto-generate SESSION_SECRET if not set (zero-config Docker/manual)
If SESSION_SECRET is missing, generate a strong one on first boot and persist it
to <dataDir>/.session-secret (stable across restarts/updates). Env var still wins.
PUBLIC_BASE_URL already falls back to the request host. So Docker (B) and manual
(C) installs now run with no required .env editing. Docs + .env.example updated;
docker-compose comment translated to English.
Co-Authored-By: Claude <noreply@…>