Extract determine_shift_type into shared layer

Eliminates duplicated weekend day/night boundary logic between
the ring scheduler and Slack bot Lambdas.

src/ring-scheduler/app.py

@@ -11,21 +11,18 @@ from datetime import datetime
 from zoneinfo import ZoneInfo
 
 from shared.ring_scheduler import update_queue_routing
-from shared.schedule import FALLBACK_EXTENSION, ShiftSchedule
+from shared.schedule import (
+    FALLBACK_EXTENSION,
+    WEEKEND_DAYS,
+    ShiftSchedule,
+    determine_shift_type,
+)
 from shared.secrets import get_secret
 
 logger = logging.getLogger()
 logger.setLevel(logging.INFO)
 
 EASTERN = ZoneInfo("America/New_York")
-WEEKEND_DAYS = {"Saturday", "Sunday"}
-
-
-def determine_shift_type(now: datetime) -> str:
-    day_name = now.strftime("%A")
-    if day_name in WEEKEND_DAYS and 8 <= now.hour < 17:
-        return "day"
-    return "night"
 
 
 def handler(event, context):

src/shared/python/shared/schedule.py

@@ -8,14 +8,30 @@ Single-table design:
 """
 
 import os
+from datetime import datetime
+from zoneinfo import ZoneInfo
 
 import boto3
 from boto3.dynamodb.conditions import Key
 
-
+EASTERN = ZoneInfo("America/New_York")
+WEEKEND_DAYS = {"Saturday", "Sunday"}
 FALLBACK_EXTENSION = "100"
 
 
+def determine_shift_type(now: datetime | None = None) -> str:
+    """Return the currently active shift type: 'day' or 'night'.
+
+    Weekend 8am-5pm is the day shift; all other times are night.
+    """
+    if now is None:
+        now = datetime.now(EASTERN)
+    day_name = now.strftime("%A")
+    if day_name in WEEKEND_DAYS and 8 <= now.hour < 17:
+        return "day"
+    return "night"
+
+
 class ShiftSchedule:
     def __init__(self, table_name: str | None = None):
         dynamodb = boto3.resource("dynamodb")

src/slack-bot/app.py

@@ -16,7 +16,12 @@ from shared.blocks import (
     build_week_schedule,
 )
 from shared.ring_scheduler import update_queue_routing
-from shared.schedule import FALLBACK_EXTENSION, ShiftSchedule
+from shared.schedule import (
+    FALLBACK_EXTENSION,
+    WEEKEND_DAYS,
+    ShiftSchedule,
+    determine_shift_type,
+)
 from shared.secrets import get_secret
 
 logger = logging.getLogger(__name__)
@@ -88,16 +93,8 @@ def is_today(date_str: str) -> bool:
     return date_str == datetime.now(EASTERN).strftime("%Y-%m-%d")
 
 
-WEEKEND_DAYS = {"Saturday", "Sunday"}
-
-
 def _is_active_shift_type(shift_type: str) -> bool:
-    """Check if the given shift type is the currently active one."""
-    now = datetime.now(EASTERN)
-    day_name = now.strftime("%A")
-    if day_name in WEEKEND_DAYS and 8 <= now.hour < 17:
-        return shift_type == "day"
-    return shift_type == "night"
+    return determine_shift_type() == shift_type
 
 
 def _find_employee_shift(schedule, date_str, day_name, employee_ext):