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/models/task.py

@@ -0,0 +1,45 @@
+from sqlalchemy import Column, String, Text, Integer, Boolean, DateTime, Numeric, Enum, ForeignKey
+from sqlalchemy.orm import relationship
+from sqlalchemy.sql import func
+from app.core.database import Base
+import enum
+
+
+class Priority(str, enum.Enum):
+    LOW = "low"
+    MEDIUM = "medium"
+    HIGH = "high"
+    URGENT = "urgent"
+
+
+class Task(Base):
+    __tablename__ = "pjctrl_tasks"
+
+    id = Column(String(36), primary_key=True)
+    project_id = Column(String(36), ForeignKey("pjctrl_projects.id", ondelete="CASCADE"), nullable=False)
+    parent_task_id = Column(String(36), ForeignKey("pjctrl_tasks.id", ondelete="CASCADE"), nullable=True)
+    title = Column(String(500), nullable=False)
+    description = Column(Text, nullable=True)
+    assignee_id = Column(String(36), ForeignKey("pjctrl_users.id"), nullable=True)
+    status_id = Column(String(36), ForeignKey("pjctrl_task_statuses.id"), nullable=True)
+    priority = Column(
+        Enum("low", "medium", "high", "urgent", name="priority_enum"),
+        default="medium",
+        nullable=False
+    )
+    original_estimate = Column(Numeric(8, 2), nullable=True)
+    time_spent = Column(Numeric(8, 2), default=0, nullable=False)
+    blocker_flag = Column(Boolean, default=False, nullable=False)
+    due_date = Column(DateTime, nullable=True)
+    position = Column(Integer, default=0, nullable=False)
+    created_by = Column(String(36), ForeignKey("pjctrl_users.id"), nullable=False)
+    created_at = Column(DateTime, server_default=func.now(), nullable=False)
+    updated_at = Column(DateTime, server_default=func.now(), onupdate=func.now(), nullable=False)
+
+    # Relationships
+    project = relationship("Project", back_populates="tasks")
+    parent_task = relationship("Task", remote_side=[id], back_populates="subtasks")
+    subtasks = relationship("Task", back_populates="parent_task", cascade="all, delete-orphan")
+    assignee = relationship("User", foreign_keys=[assignee_id], back_populates="assigned_tasks")
+    creator = relationship("User", foreign_keys=[created_by], back_populates="created_tasks")
+    status = relationship("TaskStatus", back_populates="tasks")