feat(translation): Post translation columns + settings + migration (#143 step 1)

Post gains post_title_translated / description_translated / translated_source_lang
/ translation_engine_version / translated_at — filled by the translate sweep so
viewing is instant. ImportSettings gains translation_enabled (OFF by default),
interpreter_base_url (EMPTY — no default host; the operator points it at their own
Interpreter proxy behind a reverse proxy) and translation_target_lang (en),
exposed + validated via /settings/import. Migration 0083. Settings defaults +
patch + validation test.

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

tests/test_api_settings.py

@@ -28,6 +28,31 @@ async def test_patch_import_settings(client):
     assert body["transparency_threshold"] == 0.7
 
 
+@pytest.mark.asyncio
+async def test_translation_settings_defaults_and_patch(client):
+    # #143: translation is OFF by default with NO default host — the operator
+    # points it at their own Interpreter proxy and enables it.
+    body = await (await client.get("/api/settings/import")).get_json()
+    assert body["translation_enabled"] is False
+    assert body["interpreter_base_url"] == ""
+    assert body["translation_target_lang"] == "en"
+
+    ok = await client.patch("/api/settings/import", json={
+        "translation_enabled": True,
+        "interpreter_base_url": "http://interpreter.lan",
+    })
+    assert ok.status_code == 200
+    out = await ok.get_json()
+    assert out["translation_enabled"] is True
+    assert out["interpreter_base_url"] == "http://interpreter.lan"
+
+    # Type validation.
+    assert (await client.patch(
+        "/api/settings/import", json={"translation_enabled": "yes"})).status_code == 400
+    assert (await client.patch(
+        "/api/settings/import", json={"interpreter_base_url": 123})).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])