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)
+  ```

docs/api/sdk.md

@@ -1,3 +1,83 @@
 # SDK Module
 
 ::: mt5cli.sdk
+
+## Resilient multi-account orchestration
+
+The SDK ships strategy-agnostic helpers for building long-running collectors on
+top of the read-only client. None of them depend on a particular trading
+application.
+
+### Retrying transient rate collection
+
+`collect_latest_rates_for_accounts_with_retries()` wraps
+`collect_latest_rates_for_accounts()` with bounded exponential backoff. Only
+`pdmt5.Mt5TradingError` and `pdmt5.Mt5RuntimeError` are retried; the final
+failure is re-raised once `retry_count` is exhausted.
+
+```python
+from mt5cli import AccountSpec, collect_latest_rates_for_accounts_with_retries
+
+accounts = [AccountSpec(symbols=["EURUSD"], login=12345)]
+rates = collect_latest_rates_for_accounts_with_retries(
+    accounts,
+    ["M1", "H1"],
+    count=500,
+    retry_count=3,
+    backoff_base=2,  # sleeps 2s, 4s, 8s between attempts
+)
+```
+
+### Resolving credentials and `${ENV_VAR}` placeholders
+
+`resolve_account_spec()` / `resolve_account_specs()` merge explicit override
+values over `AccountSpec` fields and expand `${ENV_VAR}` placeholders, keeping
+secrets out of plan/config files. A missing environment variable raises
+`ValueError`.
+
+```python
+import os
+
+from mt5cli import AccountSpec, resolve_account_specs
+
+os.environ["MT5_LOGIN"] = "12345"
+os.environ["MT5_PASSWORD"] = "secret"
+accounts = [
+    AccountSpec(symbols=["EURUSD"], login="${MT5_LOGIN}", password="${MT5_PASSWORD}")
+]
+
+resolved = resolve_account_specs(accounts, server="Broker-Demo")
+# resolved[0].login == "12345", resolved[0].server == "Broker-Demo"
+```
+
+### Throttled incremental history updates
+
+`ThrottledHistoryUpdater` wraps `update_history()` with a minimum interval
+between successful runs (using a monotonic clock), so an application loop can
+call it every iteration without over-fetching.
+
+```python
+from pdmt5 import Mt5Config, Mt5DataClient
+
+from mt5cli import Dataset, ThrottledHistoryUpdater
+
+updater = ThrottledHistoryUpdater(
+    output="history.db",
+    datasets={Dataset.rates},
+    timeframes=["M1"],
+    interval_seconds=60,  # <= 0 updates on every call
+)
+
+client = Mt5DataClient(config=Mt5Config(login=12345))
+client.initialize_and_login_mt5()
+try:
+    while True:
+        updater.update(client, ["EURUSD", "GBPUSD"])  # no-op until 60s elapse
+        # ... do other work; break when shutting down ...
+finally:
+    client.shutdown()
+```
+
+By default `Mt5TradingError`, `Mt5RuntimeError`, and `sqlite3.Error` propagate so
+the caller controls logging; pass `suppress_errors=True` to swallow them and
+return `False` without advancing the throttle.