feat: implement task management module

Backend (FastAPI):
- Database migration for spaces, projects, task_statuses, tasks tables
- SQLAlchemy models with relationships
- Pydantic schemas for CRUD operations
- Spaces API: CRUD with soft delete
- Projects API: CRUD with auto-created default statuses
- Tasks API: CRUD, status change, assign, subtask support
- Permission middleware with Security Level filtering
- Subtask depth limit (max 2 levels)

Frontend (React + Vite):
- Layout component with navigation
- Spaces list page
- Projects list page
- Tasks list page with status management

Fixes:
- auth_client.py: use 'username' field for external API
- config.py: extend JWT expiry to 7 days
- auth/router.py: sync Redis session with JWT expiry

Tests: 36 passed (unit + integration)
E2E: All APIs verified with real authentication

OpenSpec: add-task-management archived

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

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

backend/app/core/config.py

@@ -27,7 +27,7 @@ class Settings(BaseSettings):
     # JWT
     JWT_SECRET_KEY: str = "your-secret-key-change-in-production"
     JWT_ALGORITHM: str = "HS256"
-    JWT_EXPIRE_MINUTES: int = 15
+    JWT_EXPIRE_MINUTES: int = 10080  # 7 days
 
     # External Auth API
     AUTH_API_URL: str = "https://pj-auth-api.vercel.app"