fix: address critical issues in PR #75 agent scheduling feature

This commit fixes several issues identified in the agent scheduling
feature from PR #75:

Frontend Fixes:
- Add day boundary handling in timeUtils.ts for timezone conversions
- Add utcToLocalWithDayShift/localToUTCWithDayShift functions
- Add shiftDaysForward/shiftDaysBackward helpers for bitfield adjustment
- Update ScheduleModal to correctly adjust days_of_week when crossing
  day boundaries during UTC conversion (fixes schedules running on
  wrong days for users in extreme timezones like UTC+9)

Backend Fixes:
- Add MAX_SCHEDULES_PER_PROJECT (50) limit to prevent resource exhaustion
- Wire up crash recovery callback in scheduler_service._start_agent()
- Convert schedules.py endpoints to use context manager for DB sessions
- Fix race condition in override creation with atomic delete-then-create
- Replace deprecated datetime.utcnow with datetime.now(timezone.utc)
- Add DB-level CHECK constraints for Schedule model fields

Files Modified:
- api/database.py: Add _utc_now helper, CheckConstraint imports, constraints
- progress.py: Replace deprecated datetime.utcnow
- server/routers/schedules.py: Add context manager, schedule limits
- server/services/assistant_database.py: Replace deprecated datetime.utcnow
- server/services/scheduler_service.py: Wire crash recovery, fix race condition
- ui/src/components/ScheduleModal.tsx: Use day shift functions
- ui/src/lib/timeUtils.ts: Add day boundary handling functions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/src/components/ScheduleModal.tsx

@@ -14,8 +14,9 @@ import {
   useToggleSchedule,
 } from '../hooks/useSchedules'
 import {
-  utcToLocal,
-  localToUTC,
+  utcToLocalWithDayShift,
+  localToUTCWithDayShift,
+  adjustDaysForDayShift,
   formatDuration,
   DAYS,
   isDayActive,
@@ -109,10 +110,18 @@ export function ScheduleModal({ projectName, isOpen, onClose }: ScheduleModalPro
         return
       }
 
-      // Convert local time to UTC
+      // Convert local time to UTC and get day shift
+      const { time: utcTime, dayShift } = localToUTCWithDayShift(newSchedule.start_time)
+
+      // Adjust days_of_week based on day shift
+      // If UTC is on the next day (dayShift = 1), shift days forward
+      // If UTC is on the previous day (dayShift = -1), shift days backward
+      const adjustedDays = adjustDaysForDayShift(newSchedule.days_of_week, dayShift)
+
       const scheduleToCreate = {
         ...newSchedule,
-        start_time: localToUTC(newSchedule.start_time),
+        start_time: utcTime,
+        days_of_week: adjustedDays,
       }
 
       await createSchedule.mutateAsync(scheduleToCreate)
@@ -203,8 +212,12 @@ export function ScheduleModal({ projectName, isOpen, onClose }: ScheduleModalPro
         {!isLoading && schedules.length > 0 && (
           <div className="space-y-3 mb-6 max-h-[300px] overflow-y-auto">
             {schedules.map((schedule) => {
-              const localTime = utcToLocal(schedule.start_time)
+              // Convert UTC time to local and get day shift for display
+              const { time: localTime, dayShift } = utcToLocalWithDayShift(schedule.start_time)
               const duration = formatDuration(schedule.duration_minutes)
+              // Adjust displayed days: if local is next day (dayShift=1), shift forward
+              // if local is prev day (dayShift=-1), shift backward
+              const displayDays = adjustDaysForDayShift(schedule.days_of_week, dayShift)
 
               return (
                 <div
@@ -223,7 +236,7 @@ export function ScheduleModal({ projectName, isOpen, onClose }: ScheduleModalPro
                     {/* Days */}
                     <div className="flex gap-1 mb-2">
                       {DAYS.map((day) => {
-                        const isActive = isDayActive(schedule.days_of_week, day.bit)
+                        const isActive = isDayActive(displayDays, day.bit)
                         return (
                           <span
                             key={day.label}