Move heavy GPU deps (torch, bitsandbytes) to requirements-optional.txt

torch and bitsandbytes pull in hundreds of MB of CUDA libraries
(nvidia_cublas ~423 MB) which are useless on headless servers
without an NVIDIA GPU like the NiPoGi.

- requirements.txt: keep only lightweight essential dependencies
- requirements-optional.txt (new): Orpheus TTS deps (torch, transformers,
  bitsandbytes, snac, huggingface_hub)
- Installers and update.sh: show hint about optional deps after install
- ci/package.sh: include requirements-optional.txt in tarball

requirements-optional.txt

@@ -0,0 +1,13 @@
+# Orpheus TTS 3B (plugin-tts-orpheus-3b) — GPU-heavy dependencies
+# Install only if you enable the Orpheus TTS plugin:
+#   pip install -r requirements-optional.txt
+#
+# NOTE: torch and bitsandbytes pull in CUDA libraries (~GB). On a headless
+# server (no NVIDIA GPU) these are completely unnecessary, so they are kept
+# in a separate file.
+
+torch
+transformers
+snac
+bitsandbytes
+huggingface_hub