Refactor config loading and add status code docs

- Move config loading to dedicated ConfigLoader class with validation
- Add DATABASE_MIGRATIONS.md content to TROUBLESHOOTING.md
- Add API_STATUS_CODES.md documenting all API response codes
- Update runner.csx to use new config structure
- Add check_responses.py validation script
- Update config tests for new structure

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/tests/test_startup_integration.py

@@ -17,8 +17,8 @@ def _create_test_settings(tmpdir: str) -> Settings:
         database_path=str(Path(tmpdir) / "bangui.db"),
         fail2ban_socket="/var/run/fail2ban/fail2ban.sock",
         session_secret="test-secret-12345678901234567890",
-        fail2ban_config_dir="/etc/fail2ban",
-        geoip_db_path="/usr/share/GeoIP/GeoLite2-Country.mmdb",
+        fail2ban_config_dir=tmpdir,  # Use tmpdir (exists) instead of /etc/fail2ban
+        geoip_db_path=None,  # None = skip geoip validation
         geoip_allow_http_fallback=False,
         log_level="info",
     )