refactor(models): route tasks to chat vs worker per new architecture

Chat and background model roles effectively swapped during the
conversation+curator pivot, but call sites still used OLD routing.
This commit re-routes each call to the model whose new role fits.

Moved to background_model (worker — heavy, deliberate):
- services/journal_prep.py: daily prep generation.
- services/user_profile.py: observation consolidation.

Moved to default_model (chat — small, fast):
- services/chat.py save_response_as_note: note title generation.
- services/tag_suggestions.py: tag suggestions.

Already routed correctly (unchanged): curator, closeout, consolidation,
project summaries, history summarization.

SettingsView.vue: help text rewritten for both model fields to
describe new roles. Background Model UI label renamed to Worker
Model so the heavier role is visible from the picker. Warning copy
updated to recommend OLLAMA_MAX_LOADED_MODELS=2+ so chat and worker
can stay loaded simultaneously.

Schema names default_model and background_model unchanged on purpose
(renaming requires migration + touches ~50 call sites for UX-only gain).

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

src/fabledassistant/services/chat.py

@@ -243,8 +243,15 @@ async def save_response_as_note(user_id: int, message_id: int) -> dict:
                 },
                 {"role": "user", "content": msg.content[:2000]},
             ]
-            bg_model = await get_setting(user_id, "background_model", Config.OLLAMA_BACKGROUND_MODEL)
-            title = await generate_completion(prompt_messages, bg_model)
+            # 3-8 word title generation is the kind of trivial small-input
+            # / small-output task the chat model (default_model) handles in
+            # ~1s. Routing here so the worker (background_model) isn't
+            # interrupted from heavier curator / prep / closeout passes.
+            chat_model = (
+                await get_setting(user_id, "default_model", "")
+                or Config.OLLAMA_MODEL
+            )
+            title = await generate_completion(prompt_messages, chat_model)
             title = title.strip().strip('"\'').strip()[:100]
         except Exception:
             logger.warning("Failed to generate note title, using fallback", exc_info=True)