Add resilient multi-account orchestration helpers (#22)

* Add SDK orchestration helpers for resilient multi-account collection

- collect_latest_rates_for_accounts_with_retries(): exponential-backoff
  retries around collect_latest_rates_for_accounts(), retrying only
  Mt5TradingError/Mt5RuntimeError and re-raising on exhaustion.
- resolve_account_spec()/resolve_account_specs() and
  substitute_env_placeholders(): merge explicit overrides over AccountSpec
  fields and expand ${ENV_VAR} placeholders, raising ValueError on missing
  variables.
- ThrottledHistoryUpdater: monotonic-clock throttled wrapper around
  update_history() with should_update()/update() and opt-in suppress_errors.
- load_rate_series_by_granularity(): rate-series loader keyed by
  (symbol | None, granularity_name).
- Export new APIs, add unit tests (100% coverage), and document in README
  and docs/api.

* chore: bump version from 0.5.1 to 0.5.3 (#24)

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

* fix: resolve leftover merge conflict markers in version files

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

* fix: address PR review feedback on SDK orchestration helpers

- Use single-pass env substitution to avoid TOCTOU KeyError
- Apply backoff_base to all retry delays (backoff_base ** (attempt + 1))
- Preserve integer logins in resolve_account_spec; hide login in repr
- Fix docs examples (env ordering, while True loop, backoff comment)
- Parametrize suppress_errors tests for MT5 and SQLite errors

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

---------

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

tests/test_history.py

@@ -38,6 +38,7 @@ from mt5cli.history import (
     load_incremental_start_datetimes,
     load_rate_data,
     load_rate_data_from_connection,
+    load_rate_series_by_granularity,
     load_rate_series_from_sqlite,
     parse_sqlite_timestamp,
     quote_sqlite_identifier,
@@ -2257,6 +2258,56 @@ class TestRateSourceHelpers:
         assert set(result) == {("EURUSD", 1)}
         assert len(result["EURUSD", 1]) == 2
 
+    def test_load_rate_series_by_granularity(self, tmp_path: Path) -> None:
+        """Test loading rate series keyed by symbol and granularity name."""
+        db_path = tmp_path / "granularity.db"
+        with sqlite3.connect(db_path) as conn:
+            conn.execute(
+                "CREATE TABLE rates("
+                " symbol TEXT, timeframe INTEGER, time TEXT, close REAL)",
+            )
+            conn.executemany(
+                "INSERT INTO rates(symbol, timeframe, time, close) VALUES (?, ?, ?, ?)",
+                [
+                    ("EURUSD", 1, "2024-01-01T00:00:00+00:00", 1.0),
+                    ("EURUSD", 16385, "2024-01-01T00:00:00+00:00", 1.1),
+                ],
+            )
+            create_rate_compatibility_views(conn)
+
+        result = load_rate_series_by_granularity(
+            db_path,
+            ["EURUSD"],
+            ["M1", "H1"],
+            count=1,
+        )
+
+        assert set(result) == {("EURUSD", "M1"), ("EURUSD", "H1")}
+
+    def test_load_rate_series_by_granularity_explicit_tables(
+        self,
+        tmp_path: Path,
+    ) -> None:
+        """Test explicit tables with None-symbol targets key by granularity."""
+        db_path = tmp_path / "granularity-explicit.db"
+        with sqlite3.connect(db_path) as conn:
+            conn.execute("CREATE TABLE custom_view(time TEXT, close REAL)")
+            conn.execute(
+                "INSERT INTO custom_view(time, close) VALUES (?, ?)",
+                ("2024-01-01T00:00:00+00:00", 1.0),
+            )
+
+        result = load_rate_series_by_granularity(
+            db_path,
+            [],
+            ["M1"],
+            count=1,
+            explicit_tables=["custom_view"],
+            allow_missing_symbol=True,
+        )
+
+        assert set(result) == {(None, "M1")}
+
     def test_load_rate_series_reuses_path_connection(
         self,
         tmp_path: Path,