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/core/config.py

@@ -1,4 +1,5 @@
 from pydantic_settings import BaseSettings
+from pydantic import field_validator
 from typing import List
 import os
 
@@ -24,11 +25,33 @@ class Settings(BaseSettings):
     def REDIS_URL(self) -> str:
         return f"redis://{self.REDIS_HOST}:{self.REDIS_PORT}/{self.REDIS_DB}"
 
-    # JWT
-    JWT_SECRET_KEY: str = "your-secret-key-change-in-production"
+    # JWT - Must be set in environment, no default allowed
+    JWT_SECRET_KEY: str = ""
     JWT_ALGORITHM: str = "HS256"
     JWT_EXPIRE_MINUTES: int = 10080  # 7 days
 
+    @field_validator("JWT_SECRET_KEY")
+    @classmethod
+    def validate_jwt_secret_key(cls, v: str) -> str:
+        """Validate that JWT_SECRET_KEY is set and not a placeholder."""
+        if not v or v.strip() == "":
+            raise ValueError(
+                "JWT_SECRET_KEY must be set in environment variables. "
+                "Please configure it in the .env file."
+            )
+        placeholder_values = [
+            "your-secret-key-change-in-production",
+            "change-me",
+            "secret",
+            "your-secret-key",
+        ]
+        if v.lower() in placeholder_values:
+            raise ValueError(
+                "JWT_SECRET_KEY appears to be a placeholder value. "
+                "Please set a secure secret key in the .env file."
+            )
+        return v
+
     # External Auth API
     AUTH_API_URL: str = "https://pj-auth-api.vercel.app"