fix(settings): audit pass — model auto-pull on startup, background_model empty-string bug, base URL validation

- Startup now pulls Config.OLLAMA_MODEL (system default chat model) — previously only
  embedding and background models were pulled; the primary chat model was skipped
- _warm_user_models expanded to also pull user-configured default_model and
  background_model overrides that are missing from Ollama, rather than logging and
  skipping them; pulls run before warm/KV-cache priming
- Add background_model to _MODEL_KEYS in settings route so clearing the dropdown
  deletes the DB row instead of saving "", which caused Ollama failures in tag
  suggestions, title generation, project summaries, and RSS classification
- Add http/https scheme validation to PUT /api/admin/base-url matching the CalDAV
  route pattern; a bad value no longer silently breaks invite/password-reset links
- Update admin voice config description: "Reload models" button exists to avoid
  a server restart, so the old "restart required" text was misleading

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

src/fabledassistant/routes/settings.py

@@ -65,10 +65,10 @@ async def update_settings_route():
             if installed and model not in installed:
                 return jsonify({"error": f"Model '{model}' is not installed"}), 400
 
-    # Empty string for default_model means "reset to system default".
+    # 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"})
+    _MODEL_KEYS = frozenset({"default_model", "background_model"})
     to_save = {}
     for k, v in data.items():
         str_v = str(v)