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

backend/app/api/settings.py

@@ -47,6 +47,7 @@ _EDITABLE_FIELDS = (
     "translation_enabled",
     "interpreter_base_url",
     "translation_target_lang",
+    "translation_min_confidence",
 )
 
 # Per-host external-download toggles — all plain booleans, validated uniformly.
@@ -87,6 +88,7 @@ async def get_import_settings():
         "translation_enabled": row.translation_enabled,
         "interpreter_base_url": row.interpreter_base_url,
         "translation_target_lang": row.translation_target_lang,
+        "translation_min_confidence": row.translation_min_confidence,
     })
 
 
@@ -155,6 +157,14 @@ async def update_import_settings():
     for key in ("interpreter_base_url", "translation_target_lang"):
         if key in body and not isinstance(body[key], str):
             return jsonify({"error": f"{key} must be a string"}), 400
+    # Acceptance floor (milestone 155): latin-script translations below this
+    # Interpreter confidence are kept as the original.
+    if "translation_min_confidence" in body:
+        v = body["translation_min_confidence"]
+        if not isinstance(v, (int, float)) or isinstance(v, bool) or v < 0 or v > 1:
+            return jsonify(
+                {"error": "translation_min_confidence must be a number in [0, 1]"}
+            ), 400
     if "series_suggest_threshold" in body:
         v = body["series_suggest_threshold"]
         if not isinstance(v, (int, float)) or isinstance(v, bool) or v < 0 or v > 1: