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/schemas/space.py

@@ -0,0 +1,32 @@
+from pydantic import BaseModel
+from typing import Optional
+from datetime import datetime
+
+
+class SpaceBase(BaseModel):
+    name: str
+    description: Optional[str] = None
+
+
+class SpaceCreate(SpaceBase):
+    pass
+
+
+class SpaceUpdate(BaseModel):
+    name: Optional[str] = None
+    description: Optional[str] = None
+
+
+class SpaceResponse(SpaceBase):
+    id: str
+    owner_id: str
+    is_active: bool
+    created_at: datetime
+    updated_at: datetime
+
+    class Config:
+        from_attributes = True
+
+
+class SpaceWithOwner(SpaceResponse):
+    owner_name: Optional[str] = None