fix: use DST-aware timezone for US stock price lookups (fixes winter 1-hour offset error)

[sjhddh]

Bug

price_fetcher.py hardcodes the US Eastern timezone as UTC-4 (EDT, summer):

ET_OFFSET = timedelta(hours=-4)  # EDT is UTC-4
ET_TZ = timezone(ET_OFFSET)

US Eastern time is actually UTC-5 during winter (EST) and UTC-4 during summer (EDT). The switch happens:

• EDT → EST: first Sunday of November (~5 months of the year)
• EST → EDT: second Sunday of March (~7 months of the year)

Impact: _get_us_stock_price() converts executed_at to Eastern time, then compares against Alpha Vantage's ET-labelled candle timestamps. During EST months (November – mid-March), every timestamp comparison is off by 1 hour — the function either misses an exact match or picks up the "closest previous" price from the wrong candle, silently returning an incorrect fill price for all US stock trades.

For an AI trading system that back-tests or records real trades, this introduces a systematic 1-hour price error for nearly half the calendar year.

Fix

Use zoneinfo.ZoneInfo("America/New_York") from the Python 3.9+ stdlib for proper DST-aware conversion. Fall back to a fixed UTC-5 (EST) offset on Python < 3.9 — conservative but at least correct for the majority of the year.

try:
    from zoneinfo import ZoneInfo
    _ET_ZONEINFO = ZoneInfo("America/New_York")
except ImportError:
    _ET_ZONEINFO = None  # Python < 3.9 fallback

ET_TZ = _ET_ZONEINFO if _ET_ZONEINFO is not None else timezone(timedelta(hours=-5))

No new dependencies required — zoneinfo is in the standard library.

Test plan

• Trade executed in EST period (e.g. 2025-01-15T14:30:00Z): converts to 09:30 ET (correct, not 10:30 ET)
• Trade executed in EDT period (e.g. 2025-07-15T14:30:00Z): converts to 10:30 ET (unchanged)
• DST transition edge case: 2025-03-09T06:30:00Z → 02:30 ET (just after spring-forward)