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/stores/posts.js

@@ -71,6 +71,24 @@ export const usePostsStore = defineStore('posts', () => {
     return await api.get(`/api/posts/${id}`)
   }
 
+  // Set a post's sticky translation override (auto/force/original, #155) and
+  // patch the loaded item in place with the endpoint's result (it translates /
+  // clears immediately when the service is up), so the card reflects the change
+  // without a feed reload.
+  async function applyTranslationOverride(postId, override) {
+    const res = await api.post(`/api/posts/${postId}/translation-override`, {
+      body: { override },
+    })
+    const item = items.value.find((p) => p.id === postId)
+    if (item) {
+      item.translation_override = res.translation_override
+      item.post_title_translated = res.post_title_translated
+      item.description_translated = res.description_translated
+      item.translated_source_lang = res.translated_source_lang
+    }
+    return res
+  }
+
   // Filter overlay for the around/older/newer (in-context anchored)
   // path. Keep this distinct from `filters.value` (the down-only feed)
   // so a normal-feed filter change doesn't leak into an active anchored
@@ -168,7 +186,7 @@ export const usePostsStore = defineStore('posts', () => {
   return {
     items, cursor, loading, done, error, filters,
     cursorOlder, cursorNewer, doneOlder, doneNewer, anchorId,
-    loadInitial, loadMore, getPostFull,
+    loadInitial, loadMore, getPostFull, applyTranslationOverride,
     loadAround, loadOlder, loadNewer,
   }
 })