[Bugfix] Invalidate stale ref_audio cache on local file modification (#4873) (#5670)

Signed-off-by: pranavthakur0-0 <pranavthakurdew@gmail.com>
Co-authored-by: Zhou Taichang <tzhouam@connect.ust.hk>

docs/serving/speech_api.md

@@ -131,7 +131,7 @@ Content-Type: application/json
 
 | Parameter | Type | Default | Description |
 |-----------|------|---------|-------------|
-| `ref_audio` | string | null | Reference audio (HTTP URL, base64 data URL, or `file://` URI with `--allowed-local-media-path`) |
+| `ref_audio` | string | null | Reference audio (HTTP URL, base64 data URL, or `file://` URI with `--allowed-local-media-path`). Local files fold `mtime_ns` and `size` into cache keys to automatically reload on-disk edits; HTTP URLs and base64 URIs remain cached by string locator. |
 | `ref_text` | string | null | Transcript of reference audio |
 | `x_vector_only_mode` | bool | null | Use speaker embedding only (no ICL) |