fix: surface empty OHLCV fetch as a warning, not success

Fetching a symbol the provider doesn't cover (e.g. RHM/Rheinmetall — Alpaca
serves US listings only) returned 0 bars but reported "complete · Successfully
ingested 0 records", which the UI showed as green success.

fetch_and_ingest now returns a distinct `no_data` status when the provider
returns nothing AND the ticker has no history (vs. "already up to date" when bars
exist). The fetch endpoint maps it to a `warning` source status, and the fetch
toast renders it as ⚠ with the provider message instead of success.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/api/ingestion.ts

@@ -1,7 +1,7 @@
 import apiClient from './client';
 
 export interface IngestionSourceResult {
-  status: 'ok' | 'error' | 'skipped';
+  status: 'ok' | 'error' | 'skipped' | 'warning';
   message?: string | null;
   records?: number;
   classification?: string;