feat(settings): add background model picker with KV cache performance warning

Exposes OLLAMA_BACKGROUND_MODEL as a per-user setting in General settings,
alongside the Chat Model selector. Includes an inline warning when the same
model is selected for both, explaining the KV cache performance impact.

All background task callers (title generation, tag suggestions, project
summaries, RSS classification) now read background_model from user settings,
falling back to OLLAMA_BACKGROUND_MODEL env var.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/services/chat.py

@@ -9,6 +9,7 @@ from fabledassistant.models.conversation import Conversation, Message
 from fabledassistant.config import Config
 from fabledassistant.services.llm import generate_completion
 from fabledassistant.services.notes import create_note
+from fabledassistant.services.settings import get_setting
 
 logger = logging.getLogger(__name__)
 
@@ -234,7 +235,8 @@ async def save_response_as_note(user_id: int, message_id: int) -> dict:
                 },
                 {"role": "user", "content": msg.content[:2000]},
             ]
-            title = await generate_completion(prompt_messages, Config.OLLAMA_BACKGROUND_MODEL)
+            bg_model = await get_setting(user_id, "background_model", Config.OLLAMA_BACKGROUND_MODEL)
+            title = await generate_completion(prompt_messages, bg_model)
             title = title.strip().strip('"\'').strip()[:100]
         except Exception:
             logger.warning("Failed to generate note title, using fallback", exc_info=True)