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/posts/PostCard.vue

@@ -70,6 +70,14 @@
           @click.stop="showOriginal = !showOriginal"
         >{{ showOriginal ? 'Show translation' : `Show original${sourceLangLabel}` }}</button>
 
+        <!-- Per-post translation override (#155): force a skipped translation on,
+             or keep the original for a confidently mis-flagged title. Only where
+             there's text to translate. -->
+        <PostTranslationControl
+          v-if="post.post_title || post.description_plain"
+          :post="post"
+        />
+
         <!-- Faithful (semantic) body render once expanded: backend-sanitized
              HTML (headings, lists, links, inline images). Collapsed and the
              no-detail fallback stay plain text. -->
@@ -136,6 +144,7 @@ import { useModalStore } from '../../stores/modal.js'
 import { usePostsStore } from '../../stores/posts.js'
 import { toPlainText } from '../../utils/htmlSanitize.js'
 import PostSeriesMenu from './PostSeriesMenu.vue'
+import PostTranslationControl from './PostTranslationControl.vue'
 
 const props = defineProps({
   post: { type: Object, required: true },