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

tests/test_api_settings.py

@@ -53,6 +53,26 @@ async def test_translation_settings_defaults_and_patch(client):
         "/api/settings/import", json={"interpreter_base_url": 123})).status_code == 400
 
 
+@pytest.mark.asyncio
+async def test_translation_min_confidence_default_and_patch(client):
+    # #155: the latin-script acceptance floor is an operator-tunable setting,
+    # default 0.9 (stricter than the old hardcoded 0.8).
+    body = await (await client.get("/api/settings/import")).get_json()
+    assert body["translation_min_confidence"] == 0.9
+
+    ok = await client.patch(
+        "/api/settings/import", json={"translation_min_confidence": 0.95}
+    )
+    assert ok.status_code == 200
+    assert (await ok.get_json())["translation_min_confidence"] == 0.95
+
+    # Out-of-range + wrong-type are rejected.
+    assert (await client.patch(
+        "/api/settings/import", json={"translation_min_confidence": 1.5})).status_code == 400
+    assert (await client.patch(
+        "/api/settings/import", json={"translation_min_confidence": "high"})).status_code == 400
+
+
 @pytest.mark.asyncio
 async def test_translation_status_defaults(client):
     # Off + no URL → no network health call, healthy False (#143).