Let admins hear a voice sample before picking it

The Speech section of every model settings form now has "Hear a
sample", which streams a short sentence (AppData.VoicePreviewText) in
the form's model and voice before they're saved, through
GET api/admin/harness/preview-voice (super admins and admins, that
model only via grabModelCandidate). pickVoice is shared with
speakText, and explainAudioError reads why a streamed player failed
for both read-aloud and the sample.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/schemas/harness-schema.ts

@@ -90,3 +90,10 @@ export type SaveHarnessSettingsRequest = z.infer<
 export type SaveBotHarnessSettingsRequest = z.infer<
     typeof SaveBotHarnessSettingsRequestSchema
 >;
+
+export const PreviewVoiceRequestSchema = z.object({
+    model_id: z.coerce.number().int().positive("Pick a speech model"),
+    voice: z.string().trim().max(200).optional(),
+});
+
+export type PreviewVoiceRequest = z.infer<typeof PreviewVoiceRequestSchema>;