app_config: reject non-object config.json sections with AppConfigError

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VPNF37MP9Kuv62B1VBrZ1b

app_config.py

@@ -66,6 +66,8 @@ class AppConfig:
 
 
 def _check_keys(section: Dict[str, Any], known: set, where: str) -> None:
+    if not isinstance(section, dict):
+        raise AppConfigError(f"config.json: {where} must be a JSON object")
     unknown = sorted(set(section) - known)
     if unknown:
         raise AppConfigError(
@@ -141,6 +143,8 @@ def load_app_config(path: str = CONFIG_PATH) -> AppConfig:
         app.scan_timeframes = [Timeframe[n] for n in names]
 
     pair_section = raw.get("pairs", {})
+    if not isinstance(pair_section, dict):
+        raise AppConfigError('config.json: "pairs" must be a JSON object')
     for symbol, overrides in pair_section.items():
         if symbol not in PAIR_INFO:
             raise AppConfigError(