Replace resemble-perth with a no-op stub

resemble-perth uses uv-build which is incompatible with the old system
pip in the ROCm base image. Since watermarking is unnecessary for
self-hosted private use, stub out the perth module so chatterbox's
import is satisfied without any build complexity.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

perth_stub.py

@@ -0,0 +1,11 @@
+# Stub for resemble-perth audio watermarking.
+# Watermarking is unnecessary for self-hosted/private deployments.
+# This stub satisfies chatterbox's import without the build complexity.
+
+class PerthImplicitWatermarker:
+    def apply_watermark(self, audio, sample_rate):
+        return audio
+
+class PerthImperceptibleWatermarker:
+    def apply_watermark(self, audio, sample_rate):
+        return audio