ci: inject API keys from Gitea Secrets to backend tests

Add environment variables to backend test job:
- KRX_OPENAPI: Korea Exchange API key
- OPENDART_API: OpenDart financial data API key
- KIS_APP_KEY: Korea Investment & Securities API key

Enables tests to use real market data instead of stub data.
Secrets configured in Gitea repository settings.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -60,6 +60,9 @@ jobs:
           --blame-hang --blame-hang-timeout 2m
         env:
           KARTSELL_POSTGRES: Host=postgres;Port=5432;Database=kartsell;Username=kartsell;Password=kartsell
+          KRX_OPENAPI: ${{ secrets.KRX_OPENAPI }}
+          OPENDART_API: ${{ secrets.OPENDART_API }}
+          KIS_APP_KEY: ${{ secrets.KIS_APP_KEY }}
 
       - name: Check OpenAPI Breaking Changes (AEG-X-008)
         run: |