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/chat.py

@@ -135,7 +135,7 @@ async def send_message_route(conv_id: int):
         if msg.role != "system":
             history.append({"role": msg.role, "content": msg.content})
 
-    model = conv.model or await get_setting(uid, "default_model", Config.OLLAMA_MODEL)
+    model = conv.model or await get_setting(uid, "default_model", Config.OLLAMA_MODEL) or Config.OLLAMA_MODEL
 
     # Launch background generation task (context building happens inside the task)
     asyncio.create_task(run_generation(
@@ -257,7 +257,7 @@ async def summarize_conversation_route(conv_id: int):
     conv = await get_conversation(uid, conv_id)
     if conv is None:
         return jsonify({"error": "Conversation not found"}), 404
-    model = conv.model or await get_setting(uid, "default_model", Config.OLLAMA_MODEL)
+    model = conv.model or await get_setting(uid, "default_model", Config.OLLAMA_MODEL) or Config.OLLAMA_MODEL
     try:
         note = await summarize_conversation_as_note(uid, conv_id, model)
         return jsonify(note), 201
@@ -319,6 +319,9 @@ async def chat_status_route():
     """Check Ollama availability, model installation, and model load state."""
     uid = get_current_user_id()
     default_model = await get_setting(uid, "default_model", Config.OLLAMA_MODEL)
+    # Guard against empty-string rows written by older code when user selected "default"
+    if not default_model:
+        default_model = Config.OLLAMA_MODEL
     result = {
         "ollama": "unavailable",
         "model": "not_found",