feat(translation): 8h sweep + drain button + detection probe (#1376)

Throughput: translate_posts now runs every 8h (was daily) as the
steady-state cadence for newly-imported posts, and the Settings
"Translate now" button runs it in drain mode (run-until-done, no reset)
so one press clears the whole untranslated backlog instead of a single
300-post chunk. The interrupt/backoff re-enqueue now preserves the drain
flag so a bulk drain resumes cleanly after an Interpreter restart.

Misdetection groundwork: surface the detector's confidence from the
Interpreter client (it was in the detectedLanguage payload but discarded)
and add a read-only "Test translation" box — POST /settings/translation/
probe + TranslationCard UI — that shows detected language + confidence +
engine + result for pasted text, without saving. Lets the operator see
why a short/abbreviation-heavy English title gets mis-detected so the
detection guard (min-length + confidence floor) can be tuned from real
numbers. The guard itself follows once the mis-detected cases are probed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CgZP9v2otxVJymiYsnVuMy

frontend/src/components/settings/TranslationCard.vue

@@ -78,6 +78,61 @@
       artist, use the Re-translate button on that artist's page.
     </p>
 
+    <v-divider class="my-3" />
+
+    <!-- Diagnostic probe (task #1376): paste text → what Interpreter DETECTS
+         (language + confidence) and returns, without saving. Used to see why a
+         short/abbreviation-heavy English title gets mis-detected, and to pick a
+         detection-confidence / min-length guard from real numbers. -->
+    <div class="mb-1">
+      <span class="fc-section-h">Test translation</span>
+      <p class="fc-muted text-caption mt-1 mb-2">
+        Paste a title or snippet to see what Interpreter detects and returns —
+        handy for checking why a short string is mis-detected. Nothing is saved.
+      </p>
+    </div>
+    <v-textarea
+      v-model="probeText" label="Text to test" rows="2" auto-grow
+      density="compact" hide-details class="mb-2" :disabled="!baseUrl"
+    />
+    <div class="d-flex align-center flex-wrap mb-2" style="gap: 8px;">
+      <v-btn
+        size="small" variant="tonal" rounded="pill" prepend-icon="mdi-magnify"
+        :loading="probing" :disabled="!baseUrl || !probeText.trim()"
+        @click="onProbe"
+      >Test translation</v-btn>
+    </div>
+    <v-sheet
+      v-if="probeResult" rounded class="pa-3 mb-1 text-body-2"
+      color="rgba(var(--v-theme-accent), 0.06)"
+    >
+      <div class="d-flex flex-wrap" style="gap: 4px 16px;">
+        <span>
+          <span class="fc-muted">Detected:</span>
+          <strong>{{ probeResult.detected_lang || '—' }}</strong>
+        </span>
+        <span v-if="probeResult.detected_confidence != null">
+          <span class="fc-muted">Confidence:</span>
+          <strong>{{ fmtConf(probeResult.detected_confidence) }}</strong>
+        </span>
+        <span>
+          <span class="fc-muted">Engine:</span>
+          {{ probeResult.engine || '—' }}<template
+            v-if="probeResult.engine_version"
+          > ({{ probeResult.engine_version }})</template>
+        </span>
+        <span><span class="fc-muted">Target:</span> {{ probeResult.target }}</span>
+      </div>
+      <v-divider class="my-2" />
+      <div class="fc-muted mb-1">
+        Result<template v-if="probeResult.detected_lang === probeResult.target">
+          — already {{ probeResult.target }}, so the sweep leaves it as-is</template>:
+      </div>
+      <p class="mb-0" style="white-space: pre-wrap;">{{
+        probeResult.translated || '(no change)'
+      }}</p>
+    </v-sheet>
+
     <v-alert
       v-if="err" type="error" variant="tonal" density="compact"
       class="mt-3" closable @click:close="err = null"
@@ -129,6 +184,17 @@ const testResult = ref(null)   // null = not tested this session; true/false = l
 const status = ref(null)
 const err = ref(null)
 
+// #1376 "Test translation" probe: paste text → detected lang/confidence + result.
+const probeText = ref('')
+const probing = ref(false)
+const probeResult = ref(null)
+
+// Confidence scale is Interpreter-defined (0–100 or 0–1) — show it as-is, just
+// trimmed to 2 decimals, so the operator reads the true number.
+function fmtConf (c) {
+  return c == null ? '' : Math.round(c * 100) / 100
+}
+
 const statusColor = computed(() => {
   if (!enabled.value || !baseUrl.value) return 'grey'
   return status.value?.healthy ? 'success' : 'error'
@@ -170,6 +236,21 @@ async function onTest() {
   }
 }
 
+async function onProbe () {
+  probing.value = true
+  err.value = null
+  probeResult.value = null
+  try {
+    probeResult.value = await api.post('/api/settings/translation/probe', {
+      body: { text: probeText.value.trim() },
+    })
+  } catch (e) {
+    err.value = e.message
+  } finally {
+    probing.value = false
+  }
+}
+
 onMounted(async () => {
   try {
     await store.loadSettings()