feat: implement collaboration module

- Backend (FastAPI):
  - Task comments with nested replies and soft delete
  - @mention parsing with 10-mention limit per comment
  - Notification system with read/unread tracking
  - Blocker management with project owner notification
  - WebSocket endpoint with JWT auth and keepalive
  - User search API for @mention autocomplete
  - Alembic migration for 4 new tables

- Frontend (React + Vite):
  - Comments component with @mention autocomplete
  - NotificationBell with real-time WebSocket updates
  - BlockerDialog for task blocking workflow
  - NotificationContext for state management

- OpenSpec:
  - 4 requirements with scenarios defined
  - add-collaboration change archived

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

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

backend/app/models/notification.py

@@ -0,0 +1,37 @@
+import uuid
+from sqlalchemy import Column, String, Text, Boolean, DateTime, ForeignKey, Enum
+from sqlalchemy.sql import func
+from sqlalchemy.orm import relationship
+from app.core.database import Base
+import enum
+
+
+class NotificationType(str, enum.Enum):
+    MENTION = "mention"
+    ASSIGNMENT = "assignment"
+    BLOCKER = "blocker"
+    STATUS_CHANGE = "status_change"
+    COMMENT = "comment"
+    BLOCKER_RESOLVED = "blocker_resolved"
+
+
+class Notification(Base):
+    __tablename__ = "pjctrl_notifications"
+
+    id = Column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    user_id = Column(String(36), ForeignKey("pjctrl_users.id", ondelete="CASCADE"), nullable=False)
+    type = Column(
+        Enum("mention", "assignment", "blocker", "status_change", "comment", "blocker_resolved",
+             name="notification_type_enum"),
+        nullable=False
+    )
+    reference_type = Column(String(50), nullable=False)
+    reference_id = Column(String(36), nullable=False)
+    title = Column(String(200), nullable=False)
+    message = Column(Text, nullable=True)
+    is_read = Column(Boolean, default=False, nullable=False)
+    created_at = Column(DateTime, server_default=func.now(), nullable=False)
+    read_at = Column(DateTime, nullable=True)
+
+    # Relationships
+    user = relationship("User", back_populates="notifications")