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>

server/services/assistant_database.py

@@ -7,7 +7,7 @@ Each project has its own assistant.db file in the project directory.
 """
 
 import logging
-from datetime import datetime
+from datetime import datetime, timezone
 from pathlib import Path
 from typing import Optional
 
@@ -19,6 +19,11 @@ logger = logging.getLogger(__name__)
 Base = declarative_base()
 
 
+def _utc_now() -> datetime:
+    """Return current UTC time. Replacement for deprecated datetime.utcnow()."""
+    return datetime.now(timezone.utc)
+
+
 class Conversation(Base):
     """A conversation with the assistant for a project."""
     __tablename__ = "conversations"
@@ -26,8 +31,8 @@ class Conversation(Base):
     id = Column(Integer, primary_key=True, index=True)
     project_name = Column(String(100), nullable=False, index=True)
     title = Column(String(200), nullable=True)  # Optional title, derived from first message
-    created_at = Column(DateTime, default=datetime.utcnow)
-    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+    created_at = Column(DateTime, default=_utc_now)
+    updated_at = Column(DateTime, default=_utc_now, onupdate=_utc_now)
 
     messages = relationship("ConversationMessage", back_populates="conversation", cascade="all, delete-orphan")
 
@@ -40,7 +45,7 @@ class ConversationMessage(Base):
     conversation_id = Column(Integer, ForeignKey("conversations.id"), nullable=False, index=True)
     role = Column(String(20), nullable=False)  # "user" | "assistant" | "system"
     content = Column(Text, nullable=False)
-    timestamp = Column(DateTime, default=datetime.utcnow)
+    timestamp = Column(DateTime, default=_utc_now)
 
     conversation = relationship("Conversation", back_populates="messages")
 
@@ -175,7 +180,7 @@ def add_message(project_dir: Path, conversation_id: int, role: str, content: str
         session.add(message)
 
         # Update conversation's updated_at timestamp
-        conversation.updated_at = datetime.utcnow()
+        conversation.updated_at = _utc_now()
 
         # Auto-generate title from first user message if not set
         if not conversation.title and role == "user":

server/services/scheduler_service.py

@@ -368,6 +368,14 @@ class SchedulerService:
             logger.info(f"Agent already running for {project_name}, skipping scheduled start")
             return
 
+        # Register crash callback to enable auto-restart during scheduled windows
+        async def on_status_change(status: str):
+            if status == "crashed":
+                logger.info(f"Crash detected for {project_name}, attempting recovery")
+                await self.handle_crash_during_window(project_name, project_dir)
+
+        manager.add_status_callback(on_status_change)
+
         logger.info(
             f"Starting agent for {project_name} "
             f"(schedule {schedule.id}, yolo={schedule.yolo_mode}, concurrency={schedule.max_concurrency})"
@@ -382,6 +390,8 @@ class SchedulerService:
             logger.info(f"✓ Agent started successfully for {project_name}")
         else:
             logger.error(f"✗ Failed to start agent for {project_name}: {msg}")
+            # Remove callback if start failed
+            manager.remove_status_callback(on_status_change)
 
     async def _stop_agent(self, project_name: str, project_dir: Path):
         """Stop the agent for a project."""
@@ -457,7 +467,10 @@ class SchedulerService:
     def _create_override_for_active_schedules(
         self, project_name: str, project_dir: Path, override_type: str
     ):
-        """Create overrides for all active schedule windows."""
+        """Create overrides for all active schedule windows.
+
+        Uses atomic delete-then-create pattern to prevent race conditions.
+        """
         from api.database import Schedule, ScheduleOverride, create_database
 
         try:
@@ -479,17 +492,20 @@ class SchedulerService:
                     # Calculate window end time
                     window_end = self._calculate_window_end(schedule, now)
 
-                    # Check if override already exists
-                    existing = db.query(ScheduleOverride).filter(
+                    # Atomic operation: delete any existing overrides of this type
+                    # and create a new one in the same transaction
+                    deleted = db.query(ScheduleOverride).filter(
                         ScheduleOverride.schedule_id == schedule.id,
                         ScheduleOverride.override_type == override_type,
-                        ScheduleOverride.expires_at > now,
-                    ).first()
+                    ).delete()
 
-                    if existing:
-                        continue
+                    if deleted:
+                        logger.debug(
+                            f"Removed {deleted} existing '{override_type}' override(s) "
+                            f"for schedule {schedule.id}"
+                        )
 
-                    # Create override
+                    # Create new override
                     override = ScheduleOverride(
                         schedule_id=schedule.id,
                         override_type=override_type,