Add strategy-aware Firstrade session check policy

[Pigbibi]

Summary

• add FIRSTRADE_SESSION_CHECK_POLICY with auto/always/skip handling for /session-check
• in auto mode, throttle monthly snapshot strategies to one real maintenance check per calendar month when GCS state is available
• keep daily and daily-canary strategies running every scheduler hit, and run conservatively when state or strategy context is unavailable
• document the new env var and add session-check policy tests

Validation

• uv run --no-project --with pytest --with pandas --with requests --with google-cloud-storage --with firstrade==0.0.38 pytest tests/test_runtime_broker_adapters.py tests/test_firstrade_client.py tests/test_rebalance_service.py tests/test_session_check_service.py -q
• uv run --no-project --with ruff ruff check application/session_check_service.py tests/test_session_check_service.py main.py

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Scope auto session-check markers per account identity

When FIRSTRADE_ACCOUNT is unset (which is a supported single-account setup), every deployment for the same strategy writes and reads session-checks/auto/.... If two Firstrade credentials share the same GCS bucket/prefix and strategy, the first account's monthly marker causes the other account's /session-check to skip without ever connecting, so its session/account maintenance is suppressed for the rest of the period. Include a stable credential/account scope in the key even for the auto account case, or avoid skipping when the account scope is ambiguous.

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