feat: add week boundary computation with DST handling

Made-with: Cursor

src/ntr_fetcher/week.py

@@ -0,0 +1,38 @@
+from datetime import datetime, timedelta, timezone
+from zoneinfo import ZoneInfo
+
+EASTERN = ZoneInfo("America/New_York")
+SHOW_DAY_DEFAULT = 2  # Wednesday (Monday=0)
+SHOW_HOUR_DEFAULT = 22
+
+
+def get_show_week(
+    now_utc: datetime,
+    show_day: int = SHOW_DAY_DEFAULT,
+    show_hour: int = SHOW_HOUR_DEFAULT,
+) -> tuple[datetime, datetime]:
+    """Return (week_start_utc, week_end_utc) for the show week containing now_utc.
+
+    The week starts at show_day at show_hour Eastern Time and runs for 7 days.
+    """
+    now_et = now_utc.astimezone(EASTERN)
+
+    days_since_show_day = (now_et.weekday() - show_day) % 7
+    candidate_date = now_et.date() - timedelta(days=days_since_show_day)
+    candidate = datetime(
+        candidate_date.year,
+        candidate_date.month,
+        candidate_date.day,
+        show_hour,
+        0,
+        0,
+        tzinfo=EASTERN,
+    )
+
+    if candidate > now_et:
+        candidate -= timedelta(days=7)
+
+    week_start_utc = candidate.astimezone(timezone.utc).replace(tzinfo=timezone.utc)
+    week_end_utc = (candidate + timedelta(days=7)).astimezone(timezone.utc).replace(tzinfo=timezone.utc)
+
+    return week_start_utc, week_end_utc