feat(translation): re-translate on model change — artist-scoped + global re-run (#146)

retranslate_posts resets the 5 translation columns to NULL for a scoped set of posts (all, or WHERE artist_id IN ids) then reuses the untranslated sweep to re-run them, chasing the tail until drained (run-until-done). Interpreter cache keys on engine_version so a changed model re-translates, an unchanged one is cache-fast. Reset only happens when the service is configured+healthy so translations are never wiped when they can't be rebuilt. New POST /settings/translation/retranslate (artist_id | all=true). UI: per-artist 'Re-translate posts' on the Artist Management tab + 'Re-translate all' in the Settings Translation card, both with confirm dialogs. No migration (reuses m143 columns).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_api_settings.py

@@ -97,6 +97,68 @@ async def test_translation_run_enqueues_when_configured(client, monkeypatch):
     assert (await resp.get_json())["celery_task_id"] == "t1"
 
 
+def _fake_retranslate(monkeypatch, sink):
+    # Record the kwargs the endpoint enqueues with, return a fake AsyncResult.
+    monkeypatch.setattr(
+        "backend.app.tasks.translation.retranslate_posts.delay",
+        lambda *a, **k: sink.update(k) or type("R", (), {"id": "r1"})(),
+    )
+
+
+@pytest.mark.asyncio
+async def test_translation_retranslate_requires_config(client):
+    # Disabled → 400 even with an explicit scope (no reset is enqueued).
+    resp = await client.post(
+        "/api/settings/translation/retranslate", json={"all": True})
+    assert resp.status_code == 400
+
+
+@pytest.mark.asyncio
+async def test_translation_retranslate_requires_target(client, monkeypatch):
+    # 'all' must be explicit: an empty body is a 400, so a stray call can't
+    # wipe every translation.
+    _fake_retranslate(monkeypatch, {})
+    await client.patch("/api/settings/import", json={
+        "translation_enabled": True, "interpreter_base_url": "http://i.lan",
+    })
+    resp = await client.post("/api/settings/translation/retranslate")
+    assert resp.status_code == 400
+
+
+@pytest.mark.asyncio
+async def test_translation_retranslate_artist_scope(client, monkeypatch):
+    sink = {}
+    _fake_retranslate(monkeypatch, sink)
+    await client.patch("/api/settings/import", json={
+        "translation_enabled": True, "interpreter_base_url": "http://i.lan",
+    })
+    resp = await client.post(
+        "/api/settings/translation/retranslate", json={"artist_id": 7})
+    assert resp.status_code == 202
+    assert (await resp.get_json())["celery_task_id"] == "r1"
+    assert sink["artist_ids"] == [7]
+
+
+@pytest.mark.asyncio
+async def test_translation_retranslate_all_scope(client, monkeypatch):
+    sink = {}
+    _fake_retranslate(monkeypatch, sink)
+    await client.patch("/api/settings/import", json={
+        "translation_enabled": True, "interpreter_base_url": "http://i.lan",
+    })
+    resp = await client.post(
+        "/api/settings/translation/retranslate", json={"all": True})
+    assert resp.status_code == 202
+    assert sink["artist_ids"] is None
+
+
+@pytest.mark.asyncio
+async def test_translation_retranslate_bad_artist_id(client):
+    resp = await client.post(
+        "/api/settings/translation/retranslate", json={"artist_id": "abc"})
+    assert resp.status_code == 400
+
+
 @pytest.mark.asyncio
 async def test_patch_rejects_non_object(client):
     resp = await client.patch("/api/settings/import", json=[1, 2, 3])