ux: rename model fields + enforce serial curator execution

Three coordinated changes per operator request 2026-05-24:

1. Settings UI rename matching the language we actually use:
   - Chat Model -> Chat & Voice Model
   - Worker Model -> Curator Model
   Setting KEYS (default_model / background_model) unchanged on
   purpose; renaming them requires a migration touching 50+ call
   sites for purely UX-facing benefit.

2. Settings UI help text rewritten:
   - Chat & Voice: documents that it handles chat AND small
     conversational automations (titles, tags). Recommends
     OLLAMA_NUM_PARALLEL=2+ on the Ollama server so background
     automations get their own KV-cache slot and don't evict
     the chat model's working state.
   - Curator: notes the app enforces SERIAL execution regardless
     of NUM_PARALLEL — only one curator pass runs at a time. This
     matters most for 70b CPU models where a second instance
     would waste system RAM.

3. Enforce serial curator execution globally:
   - New module-level _CURATOR_RUN_LOCK in services/curator.py.
   - run_curator_for_conversation now wraps its body in 'async
     with _CURATOR_RUN_LOCK' — every entry point (scheduler sweep,
     manual route trigger, future hooks) is serialized through it.
   - is_curator_running() helper exposes the lock state.
   - routes/journal.py manual trigger checks is_curator_running()
     first and returns 409 {busy: true} immediately rather than
     blocking the HTTP request for minutes waiting for a 70b CPU
     pass to finish. The user can retry once the curator clears.

   Why a 409 instead of queue: a curator pass on a 70b CPU model
   can take 5+ minutes. Tying up an HTTP worker that long is bad;
   making the user wait without feedback is worse. 409 surfaces
   the busy state immediately and the user retries when they want.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/fabledassistant/routes/journal.py

@@ -199,7 +199,19 @@ async def trigger_curator_run(conv_id: int):
         if _res.scalar_one_or_none() is None:
             return jsonify({"error": "Conversation not found"}), 404
 
-    from fabledassistant.services.curator import run_curator_for_conversation
+    from fabledassistant.services.curator import (
+        is_curator_running,
+        run_curator_for_conversation,
+    )
+    # The curator typically runs on a large model (30b-70b on CPU); we
+    # serialize runs globally via a module-level lock. Reject rather
+    # than block when busy — blocking would tie up an HTTP worker for
+    # minutes. The user can retry in a moment.
+    if is_curator_running():
+        return jsonify({
+            "error": "Curator is currently running. Please try again in a moment.",
+            "busy": True,
+        }), 409
     result = await run_curator_for_conversation(conv_id)
 
     # Stamp last_curator_run_at on success so the scheduler doesn't