[Frontend] Reject built-in name collisions in voice upload and add VLLM_OMNI_SPEAKER_REGISTRATION_POLICY (#6848)

Signed-off-by: Hoseung Kim <hoseung.kim@navercorp.com>
Co-authored-by: Hoseung Kim <hoseung.kim@navercorp.com>

docs/serving/speech_api.md

@@ -289,6 +289,19 @@ Upload a new voice sample for voice cloning in Base task TTS requests.
 
 Fields `ref_text` and `speaker_description` are omitted when not provided at upload time.
 
+**Naming rules:**
+
+- Names that collide with one of the model's built-in or precomputed voices are rejected (400). Voice
+  files already on disk under such a name are ignored at startup with a warning, so an upload can never
+  shadow a built-in voice.
+- Re-uploading an existing uploaded name overwrites it in place (the previous audio file is
+  deleted). Set `VLLM_OMNI_SPEAKER_REGISTRATION_POLICY=immutable` on the server to reject duplicates instead,
+  requiring an explicit `DELETE /v1/audio/voices/{name}` before re-registering — useful when the
+  endpoint is reachable by multiple writers and silent overwrites are a risk.
+- The voice registry has no per-user ownership: any client that can reach the endpoint can
+  overwrite (default policy) or delete any uploaded voice. For multi-tenant deployments, add
+  authentication at a proxy and namespace voice names per user (e.g. `{user}.{name}`).
+
 **Usage Example:**
 
 ```bash
@@ -577,6 +590,7 @@ by `GET /v1/audio/voices`. Valid precomputed voices can be used in
 | ---------- | --------- | ------------- |
 | `SPEAKER_SAMPLES_DIR` | `~/.cache/vllm-omni/speakers` | Directory for persisted uploaded speakers (`.safetensors` files). |
 | `SPEAKER_MAX_UPLOADED` | `1000` | Maximum number of uploaded speakers kept on disk. Upload requests past the cap return 400. |
+| `VLLM_OMNI_SPEAKER_REGISTRATION_POLICY` | `overwrite` | `immutable` rejects re-uploading an existing uploaded name (400) until it is deleted; any other value fails startup. |
 
 The in-memory LRU has a fixed 512 MiB byte budget.