feat(translation): expose translated fields in post feed + provenance payloads (#143 step 4)

post_feed_service (card + detail) and provenance_service._post_dict now include
post_title_translated, description_translated (card-truncated / detail-uncapped)
and translated_source_lang, keeping the originals for the toggle. Feed
serialization test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

tests/test_post_feed_service.py

@@ -283,6 +283,30 @@ async def test_scroll_item_shape_minimal(db):
     assert "description_full" not in item
 
 
+@pytest.mark.asyncio
+async def test_scroll_surfaces_translation_fields(db):
+    # #143: a translated post exposes the translated title/description + source
+    # lang, with the originals still present for the toggle.
+    artist = await _seed_artist(db, "alice-tr")
+    src = await _seed_source(db, artist.id, "pixiv", "https://p/alice-tr")
+    p = await _seed_post(
+        db, src.id, external_id="TR", post_date=datetime.now(UTC),
+        title="ねこ", description="<p>かわいい</p>",
+    )
+    p.post_title_translated = "cat"
+    p.description_translated = "cute"
+    p.translated_source_lang = "ja"
+    await db.commit()
+
+    page = await PostFeedService(db).scroll(cursor=None, limit=10)
+    item = page["items"][0]
+    assert item["post_title_translated"] == "cat"
+    assert item["description_translated"] == "cute"
+    assert item["translated_source_lang"] == "ja"
+    assert item["post_title"] == "ねこ"           # original kept for the toggle
+    assert item["description_plain"] == "かわいい"
+
+
 @pytest.mark.asyncio
 async def test_scroll_description_truncates_long_text(db):
     artist = await _seed_artist(db, "alice-long")