feat: add injectable update_backend to ThrottledHistoryUpdater (#38)

* feat: add injectable update_backend to ThrottledHistoryUpdater

Allow downstream applications to substitute the history update backend via
the ThrottledHistoryUpdater constructor without monkey-patching
mt5cli.sdk.update_history. Defaults to update_history for backward
compatibility.

Co-authored-by: Daichi Narushima <dceoy@users.noreply.github.com>

* chore: fix lint and format after QA

Co-authored-by: Daichi Narushima <dceoy@users.noreply.github.com>

* chore: bump version to 0.8.2

Co-authored-by: Daichi Narushima <dceoy@users.noreply.github.com>

* fix: use explicit None check for ThrottledHistoryUpdater backend

Only None selects the default update_history backend so falsy callable
objects with __bool__ returning False are preserved as custom backends.

Co-authored-by: Daichi Narushima <dceoy@users.noreply.github.com>

---------

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Daichi Narushima <dceoy@users.noreply.github.com>

docs/api/sdk.md

@@ -113,6 +113,28 @@ finally:
     client.shutdown()
 ```
 
+Pass `update_backend` to substitute the default `update_history` implementation
+without monkey-patching `mt5cli.sdk.update_history`. The callable receives the
+same keyword arguments as `update_history` (`client`, `output`, `symbols`,
+`datasets`, `timeframes`, `flags`, `lookback_hours`, `with_views`,
+`include_account_events`). The resolved backend is stored on
+`updater.update_backend` for inspection or subclassing.
+
+```python
+from mt5cli import ThrottledHistoryUpdater, update_history
+
+
+def app_update_history(**kwargs) -> None:
+    update_history(**kwargs)  # or delegate to application-specific logic
+
+
+updater = ThrottledHistoryUpdater(
+    output="history.db",
+    interval_seconds=60,
+    update_backend=app_update_history,
+)
+```
+
 By default recoverable errors (`Mt5TradingError`, `Mt5RuntimeError`,
 `sqlite3.Error`, `ValueError`, `OSError`, and MT5 client capability
 `AttributeError` / `TypeError` for history API methods) propagate so the caller