Add real exchange integration tests for three-tier cache system

Comprehensive tests that fetch actual data from Binance to verify:
- Three-tier caching (memory → database → exchange)
- Gap detection and filling (start, end, middle, multiple gaps)
- Data integrity verification against direct exchange fetch
- Freshness checking for stale data
- Different timeframe handling
- Edge cases (single candle, large ranges, overlapping requests)

Tests are marked with @real_exchange and can be skipped in CI with:
  pytest -m "not real_exchange"

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

pyproject.toml

@@ -36,3 +36,6 @@ where = ["src"]
 [tool.pytest.ini_options]
 testpaths = ["tests"]
 asyncio_mode = "auto"
+markers = [
+    "real_exchange: tests that require real exchange connectivity (deselect with '-m \"not real_exchange\"')",
+]