Fix 'default' model selection breaking readiness indicator

When a user selected the 'Default' option in Settings, the dropdown
sent an empty string "" to the backend. The route saved it as a DB row,
which caused get_setting() to return "" instead of falling back to
Config defaults. The chat status endpoint then tried to match "" against
installed model names — always failing — resulting in model: "not_found"
and a permanently failing readiness indicator.

services/settings.py:
- Add delete_setting() helper: removes a setting row so get_setting()
  correctly falls back to its hardcoded default argument

routes/settings.py:
- Import delete_setting
- When default_model or intent_model are saved as empty string, delete
  the DB row instead of storing "" — cleanly restores Config fallback

routes/chat.py:
- chat_status_route: add explicit `or Config.OLLAMA_MODEL` guard for
  any existing "" rows written before this fix (migration safety net)
- send_message and summarize routes: same guard on model resolution
  so empty settings never cause silent generation failures

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

src/fabledassistant/routes/settings.py

@@ -4,7 +4,7 @@ from fabledassistant.auth import login_required, get_current_user_id
 from fabledassistant.config import Config
 from fabledassistant.services.caldav import CALDAV_SETTING_KEYS, get_caldav_config, test_connection
 from fabledassistant.services.llm import get_installed_models
-from fabledassistant.services.settings import get_all_settings, set_settings_batch
+from fabledassistant.services.settings import delete_setting, get_all_settings, set_settings_batch
 
 settings_bp = Blueprint("settings", __name__, url_prefix="/api/settings")
 
@@ -33,7 +33,20 @@ async def update_settings_route():
                 if installed and model not in installed:
                     return jsonify({"error": f"Model '{model}' is not installed"}), 400
 
-    await set_settings_batch(uid, {k: str(v) for k, v in data.items()})
+    # Empty string for model keys means "reset to system default".
+    # Delete the DB row so get_setting() falls back to Config defaults
+    # rather than returning "" and breaking model resolution everywhere.
+    _MODEL_KEYS = frozenset({"default_model", "intent_model"})
+    to_save = {}
+    for k, v in data.items():
+        str_v = str(v)
+        if k in _MODEL_KEYS and not str_v:
+            await delete_setting(uid, k)
+        else:
+            to_save[k] = str_v
+
+    if to_save:
+        await set_settings_batch(uid, to_save)
     settings = await get_all_settings(uid)
     return jsonify(settings)