Use central Anthropic key for AI insights; add update-check banner

AI insights now fetches the Claude API key from the central settings
service instead of storing its own encrypted copy, so wages and other
apps can share the same key. Also wires up the update-available banner
using the existing version-check hook pattern.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/services/central_settings.py

@@ -117,3 +117,17 @@ async def get_resos_credentials() -> Optional[dict]:
 def get_resos_credentials_sync() -> Optional[dict]:
     """Blocking variant of get_resos_credentials for sync job contexts."""
     return _extract_resos(get_integration_sync("resos"))
+
+
+def _extract_anthropic(s: Optional[dict]) -> Optional[dict]:
+    if not s:
+        return None
+    key = s.get("api_key") or ""
+    if not key:
+        return None
+    return {"api_key": key}
+
+
+async def get_anthropic_credentials() -> Optional[dict]:
+    """Returns {'api_key'} from central settings, or None if not configured."""
+    return _extract_anthropic(await get_integration("anthropic"))