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>

backend/app/api/settings.py

@@ -351,3 +351,39 @@ async def translation_run():
 
     r = translate_posts.delay()
     return jsonify({"celery_task_id": r.id}), 202
+
+
+@settings_bp.route("/settings/translation/retranslate", methods=["POST"])
+async def translation_retranslate():
+    """Re-translate stored translations after a model change (m146). Body:
+    ``{"artist_id": <int>}`` aims at one artist; ``{"all": true}`` re-runs every
+    artist. ``all`` must be explicit so an empty/typo body can't wipe everything.
+    Clears the scoped translations and enqueues the run-until-done retranslate
+    sweep (the Interpreter cache re-translates on a changed model, is cache-fast
+    otherwise). Same enabled + base-URL guard as 'Translate now'."""
+    body = await request.get_json(silent=True)
+    body = body if isinstance(body, dict) else {}
+    artist_id = body.get("artist_id")
+    do_all = bool(body.get("all"))
+    if artist_id is None and not do_all:
+        return jsonify(
+            {"error": "provide artist_id, or all=true to re-translate everything"}
+        ), 400
+    if artist_id is not None:
+        try:
+            artist_id = int(artist_id)
+        except (TypeError, ValueError):
+            return jsonify({"error": "artist_id must be an integer"}), 400
+
+    async with get_session() as session:
+        cfg = await ImportSettings.load(session)
+    if not cfg.translation_enabled or not (cfg.interpreter_base_url or "").strip():
+        return jsonify(
+            {"error": "translation is disabled or no base URL is set"}
+        ), 400
+    from ..tasks.translation import retranslate_posts
+
+    # artist_id wins when both are sent; otherwise all=true → None (every artist).
+    artist_ids = [artist_id] if artist_id is not None else None
+    r = retranslate_posts.delay(artist_ids=artist_ids)
+    return jsonify({"celery_task_id": r.id}), 202