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/functions/backend/db/harness/grab-model-candidate.ts

@@ -0,0 +1,63 @@
+import BunSQLite from "@moduletrace/bun-sqlite";
+import type { BUN_SQLITE_COBBLE_HARNESS_MODELS } from "@/db/types/db";
+import type { HarnessKind } from "@/src/dict/harness-dict";
+import modelServesHarness from "@/src/harness/catalog/model-serves-harness";
+import type { HarnessCandidate } from "@/src/harness/types";
+import AppError from "@/src/utils/app-error";
+import grabHarnessProviders from "./grab-harness-providers";
+import harnessModelRowToCatalog from "./harness-model-row-to-catalog";
+import resolveProviderConfig from "./resolve-provider-config";
+
+type Params = {
+    model_id: number;
+    harness: HarnessKind;
+};
+
+/**
+ * One catalog model as a candidate to call directly, whatever is saved
+ * (e.g. to try it before picking it). Throws when it's gone, can't do the
+ * harness's job, or its provider is off or has no usable key.
+ */
+export default async function grabModelCandidate({
+    model_id,
+    harness,
+}: Params): Promise<HarnessCandidate> {
+    const row = (
+        await BunSQLite.sql<BUN_SQLITE_COBBLE_HARNESS_MODELS>({
+            sql: `SELECT * FROM harness_models WHERE id = ? AND available = 1`,
+            values: [model_id],
+        })
+    ).payload?.[0];
+
+    if (!row?.provider_id) {
+        throw new AppError(`That model isn't available any more.`);
+    }
+
+    const model = harnessModelRowToCatalog({ row });
+
+    if (!modelServesHarness({ model, harness })) {
+        throw new AppError(`That model can't do this.`);
+    }
+
+    const [provider] = await grabHarnessProviders({
+        provider_id: row.provider_id,
+        enabled_only: true,
+    });
+
+    if (!provider) {
+        throw new AppError(`That model's provider is turned off.`);
+    }
+
+    const { config, usable } = await resolveProviderConfig({ provider });
+
+    if (!usable) {
+        throw new AppError(`That model's provider has no usable key.`);
+    }
+
+    return {
+        key: `${row.provider_id}:${model.slug}`,
+        provider: config,
+        model,
+        source: "pinned",
+    };
+}