perf(settings): prime KV cache when user changes chat model

When a user saves a new default_model in Settings, fire a background
cache-prime request so the first message with the new model is fast
rather than paying the full cold-start prefill cost.

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

src/fabledassistant/routes/settings.py

@@ -1,3 +1,6 @@
+import asyncio
+import logging
+
 from quart import Blueprint, jsonify, request
 
 from fabledassistant.auth import login_required, get_current_user_id
@@ -6,6 +9,35 @@ from fabledassistant.services.caldav import CALDAV_SETTING_KEYS, get_caldav_conf
 from fabledassistant.services.llm import get_installed_models, _is_private_url
 from fabledassistant.services.settings import delete_setting, get_all_settings, set_settings_batch
 
+logger = logging.getLogger(__name__)
+
+
+async def _prime_kv_cache_bg(user_id: int, model: str) -> None:
+    """Fire-and-forget: prime Ollama's KV cache with the user's system prompt."""
+    import httpx
+    from fabledassistant.services.llm import build_context
+    try:
+        messages, _ = await build_context(
+            user_id=user_id,
+            history=[],
+            current_note_id=None,
+            user_message=" ",
+        )
+        async with httpx.AsyncClient(timeout=120.0) as client:
+            await client.post(
+                f"{Config.OLLAMA_URL}/api/chat",
+                json={
+                    "model": model,
+                    "messages": messages,
+                    "stream": False,
+                    "options": {"num_predict": 1},
+                    "keep_alive": "2h",
+                },
+            )
+        logger.info("Primed KV cache for user %d with model '%s'", user_id, model)
+    except Exception:
+        logger.warning("Failed to prime KV cache for user %d", user_id, exc_info=True)
+
 settings_bp = Blueprint("settings", __name__, url_prefix="/api/settings")
 
 
@@ -46,6 +78,10 @@ async def update_settings_route():
 
     if to_save:
         await set_settings_batch(uid, to_save)
+
+    if "default_model" in to_save and to_save["default_model"]:
+        asyncio.create_task(_prime_kv_cache_bg(uid, to_save["default_model"]))
+
     settings = await get_all_settings(uid)
     return jsonify(settings)