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>

docs/api/history.md

@@ -215,3 +215,15 @@ frame = series["EURUSD", 1]  # keyed by (symbol, integer timeframe)
   requires existing managed `rate_*` compatibility views and raises
   `ValueError` when they are missing. Duplicate `(symbol, timeframe)` targets
   are rejected.
+- `load_rate_series_by_granularity()` is a thin wrapper that builds the targets,
+  loads the series, and rekeys the result by granularity name to avoid
+  converting integer timeframes downstream:
+
+  ```python
+  from mt5cli import load_rate_series_by_granularity
+
+  series = load_rate_series_by_granularity(
+      "history.db", ["EURUSD"], ["M1", "H1"], count=1000
+  )
+  frame = series["EURUSD", "M1"]  # keyed by (symbol | None, granularity_name)
+  ```