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/main.py

@@ -1,9 +1,13 @@
 from contextlib import asynccontextmanager
-from fastapi import FastAPI
+from fastapi import FastAPI, Request
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.responses import JSONResponse
+from slowapi import _rate_limit_exceeded_handler
+from slowapi.errors import RateLimitExceeded
 
 from app.middleware.audit import AuditMiddleware
 from app.core.scheduler import start_scheduler, shutdown_scheduler
+from app.core.rate_limiter import limiter
 
 
 @asynccontextmanager
@@ -29,6 +33,7 @@ from app.api.audit import router as audit_router
 from app.api.attachments import router as attachments_router
 from app.api.triggers import router as triggers_router
 from app.api.reports import router as reports_router
+from app.api.health import router as health_router
 from app.core.config import settings
 
 app = FastAPI(
@@ -38,6 +43,10 @@ app = FastAPI(
     lifespan=lifespan,
 )
 
+# Initialize rate limiter
+app.state.limiter = limiter
+app.add_exception_handler(RateLimitExceeded, _rate_limit_exceeded_handler)
+
 # CORS middleware
 app.add_middleware(
     CORSMiddleware,
@@ -66,6 +75,7 @@ app.include_router(audit_router)
 app.include_router(attachments_router)
 app.include_router(triggers_router)
 app.include_router(reports_router)
+app.include_router(health_router)
 
 
 @app.get("/health")