feat(translation): tunable acceptance floor (0.90) + per-post sticky override (#155)

The gate at a fixed 0.80 couldn't catch the real pain: Interpreter (fresh ==
cached, verified by probe) confidently mis-detects short ASCII English like
"... WIP Part 1" as German at 0.86 — above the floor — so it was accepted and a
re-translate reproduced it. Confidence alone can't separate the 0.86 collision
(genuine German lands there too), and single-word mis-flags sit at a confident
1.0 no floor catches.

Two operator-approved levers:

- Acceptance floor is now a live Settings value (ImportSettings.
  translation_min_confidence, default 0.90; surfaced in the Translation card), so
  it's tunable without a redeploy. _accept takes the threshold as a parameter.

- Per-post sticky override (Post.translation_override: auto/force/original).
  'force' stores a translation even below the floor (rescue a skipped
  legit-foreign title); 'original' keeps the original and clears any stored
  translation (kill a confident mis-flag no floor catches). The sweep honors it
  on every run and _reset_translations skips 'original', so the choice survives a
  Re-translate-all. POST /api/posts/<id>/translation-override applies it
  immediately (translate now when the service is up, else queue for the sweep).
  UI: PostTranslationControl on the posts-feed card.

Migration 0084 (both columns + a CHECK on the override). The feed + provenance
serializers expose translation_override.

With a stricter floor the rollback finally works: raise it -> Re-translate all ->
the 0.86 mis-flags are rejected and restored to the original; force /
keep-original handle the residual either way.

Tests: gate thresholds against the param (0.86 rejected at 0.90, explicit-floor
cases); sweep force/original + re-translate-skips-original; override endpoint
(validation, original clears, force queues when disabled, feed exposes it);
settings min_confidence default/save/validate.

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

@@ -28,6 +28,21 @@
       @change="onSave"
     />
 
+    <!-- Acceptance floor (#155): latin-script translations below this Interpreter
+         confidence are kept as the original. Per-post overrides handle exceptions. -->
+    <v-text-field
+      v-model.number="minConfidence" label="Acceptance confidence"
+      type="number" min="0" max="1" step="0.01" density="compact"
+      hide-details style="max-width: 200px;" class="mb-1" :disabled="busy"
+      @change="onSaveConfidence"
+    />
+    <div class="fc-muted text-caption mb-3">
+      Latin-script titles Interpreter is less than this sure about (0–1) are kept
+      as the original; CJK is always translated. Default 0.90 — raise it to reject
+      more mis-detections, lower it to translate more. Per-post controls in the
+      posts feed override this either way.
+    </div>
+
     <div class="d-flex align-center flex-wrap mb-3" style="gap: 8px;">
       <v-icon size="12" :color="statusColor">mdi-circle</v-icon>
       <span class="fc-muted text-body-2">{{ statusText }}</span>
@@ -175,6 +190,7 @@ const store = useImportStore()
 const enabled = ref(false)
 const baseUrl = ref('')
 const targetLang = ref('en')
+const minConfidence = ref(0.9)
 const busy = ref(false)
 const running = ref(false)
 const retranslating = ref(false)
@@ -258,6 +274,7 @@ onMounted(async () => {
     enabled.value = !!s.translation_enabled
     baseUrl.value = s.interpreter_base_url || ''
     targetLang.value = s.translation_target_lang || 'en'
+    minConfidence.value = s.translation_min_confidence ?? 0.9
   } catch { /* non-fatal */ }
   await loadStatus()
 })
@@ -282,6 +299,14 @@ async function onSave() {
   })
   await loadStatus()
 }
+async function onSaveConfidence() {
+  // Clamp to [0, 1] so a stray value never bounces off the API 400.
+  let v = Number(minConfidence.value)
+  if (!Number.isFinite(v)) v = 0.9
+  v = Math.min(1, Math.max(0, v))
+  minConfidence.value = v
+  await save({ translation_min_confidence: v })
+}
 async function onRun() {
   running.value = true
   err.value = null