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_posts.py

@@ -184,6 +184,68 @@ async def test_rejects_bad_direction(client):
     assert (await resp.get_json())["error"] == "invalid_direction"
 
 
+@pytest.mark.asyncio
+async def test_translation_override_rejects_bad_value(client, seeded_post):
+    _, _, post = seeded_post
+    resp = await client.post(
+        f"/api/posts/{post.id}/translation-override", json={"override": "nope"}
+    )
+    assert resp.status_code == 400
+    assert (await resp.get_json())["error"] == "invalid_override"
+
+
+@pytest.mark.asyncio
+async def test_translation_override_404_for_unknown(client):
+    resp = await client.post(
+        "/api/posts/999999/translation-override", json={"override": "original"}
+    )
+    assert resp.status_code == 404
+
+
+@pytest.mark.asyncio
+async def test_translation_override_original_clears_and_keeps(client, db, seeded_post):
+    # 'keep original' clears a stored translation immediately (no Interpreter) and
+    # marks the post handled (== target). Asserts on the response, which reflects
+    # the endpoint's own committed session.
+    _, _, post = seeded_post
+    post.post_title_translated = "STALE"
+    post.description_translated = "STALE"
+    post.translated_source_lang = "de"
+    await db.commit()
+    resp = await client.post(
+        f"/api/posts/{post.id}/translation-override", json={"override": "original"}
+    )
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["translation_override"] == "original"
+    assert body["applied"] == "cleared"
+    assert body["post_title_translated"] is None
+    assert body["translated_source_lang"] == "en"
+
+
+@pytest.mark.asyncio
+async def test_translation_override_force_queues_when_disabled(client, seeded_post):
+    # Translation disabled (default) → can't translate inline; the override is
+    # saved and the post is queued (columns NULLed) for the next sweep.
+    _, _, post = seeded_post
+    resp = await client.post(
+        f"/api/posts/{post.id}/translation-override", json={"override": "force"}
+    )
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["translation_override"] == "force"
+    assert body["applied"] == "queued"
+    assert body["translated_source_lang"] is None
+
+
+@pytest.mark.asyncio
+async def test_list_exposes_translation_override(client, seeded_post):
+    resp = await client.get("/api/posts")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["items"][0]["translation_override"] == "auto"   # default
+
+
 @pytest.mark.asyncio
 async def test_detail_returns_uncapped_thumbnails(client, db):
     """Feed query caps thumbnails at 6 for previews; detail endpoint