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>

progress.py

@@ -10,7 +10,7 @@ import json
 import os
 import sqlite3
 import urllib.request
-from datetime import datetime
+from datetime import datetime, timezone
 from pathlib import Path
 
 WEBHOOK_URL = os.environ.get("PROGRESS_N8N_WEBHOOK_URL")
@@ -171,7 +171,7 @@ def send_progress_webhook(passing: int, total: int, project_dir: Path) -> None:
             "tests_completed_this_session": passing - previous,
             "completed_tests": completed_tests,
             "project": project_dir.name,
-            "timestamp": datetime.utcnow().isoformat() + "Z",
+            "timestamp": datetime.now(timezone.utc).isoformat().replace("+00:00", "Z"),
         }
 
         try: