feat: complete issue fixes and implement remaining features

## Critical Issues (CRIT-001~003) - All Fixed
- JWT secret key validation with pydantic field_validator
- Login audit logging for success/failure attempts
- Frontend API path prefix removal

## High Priority Issues (HIGH-001~008) - All Fixed
- Project soft delete using is_active flag
- Redis session token bytes handling
- Rate limiting with slowapi (5 req/min for login)
- Attachment API permission checks
- Kanban view with drag-and-drop
- Workload heatmap UI (WorkloadPage, WorkloadHeatmap)
- TaskDetailModal integrating Comments/Attachments
- UserSelect component for task assignment

## Medium Priority Issues (MED-001~012) - All Fixed
- MED-001~005: DB commits, N+1 queries, datetime, error format, blocker flag
- MED-006: Project health dashboard (HealthService, ProjectHealthPage)
- MED-007: Capacity update API (PUT /api/users/{id}/capacity)
- MED-008: Schedule triggers (cron parsing, deadline reminders)
- MED-009: Watermark feature (image/PDF watermarking)
- MED-010~012: useEffect deps, DOM operations, PDF export

## New Files
- backend/app/api/health/ - Project health API
- backend/app/services/health_service.py
- backend/app/services/trigger_scheduler.py
- backend/app/services/watermark_service.py
- backend/app/core/rate_limiter.py
- frontend/src/pages/ProjectHealthPage.tsx
- frontend/src/components/ProjectHealthCard.tsx
- frontend/src/components/KanbanBoard.tsx
- frontend/src/components/WorkloadHeatmap.tsx

## Tests
- 113 new tests passing (health: 32, users: 14, triggers: 35, watermark: 32)

## OpenSpec Archives
- add-project-health-dashboard
- add-capacity-update-api
- add-schedule-triggers
- add-watermark-feature
- add-rate-limiting
- enhance-frontend-ux
- add-resource-management-ui

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/app/api/projects/router.py

@@ -273,9 +273,9 @@ async def delete_project(
     current_user: User = Depends(get_current_user),
 ):
     """
-    Delete a project (hard delete, cascades to tasks).
+    Delete a project (soft delete - sets is_active to False).
     """
-    project = db.query(Project).filter(Project.id == project_id).first()
+    project = db.query(Project).filter(Project.id == project_id, Project.is_active == True).first()
 
     if not project:
         raise HTTPException(
@@ -289,7 +289,7 @@ async def delete_project(
             detail="Only project owner can delete",
         )
 
-    # Audit log before deletion (this is a high-sensitivity event that triggers alert)
+    # Audit log before soft deletion (this is a high-sensitivity event that triggers alert)
     AuditService.log_event(
         db=db,
         event_type="project.delete",
@@ -297,11 +297,12 @@ async def delete_project(
         action=AuditAction.DELETE,
         user_id=current_user.id,
         resource_id=project.id,
-        changes=[{"field": "title", "old_value": project.title, "new_value": None}],
+        changes=[{"field": "is_active", "old_value": True, "new_value": False}],
         request_metadata=get_audit_metadata(request),
     )
 
-    db.delete(project)
+    # Soft delete - set is_active to False
+    project.is_active = False
     db.commit()
 
     return None