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/__init__.py

@@ -6,5 +6,12 @@ from app.models.project import Project
 from app.models.task_status import TaskStatus
 from app.models.task import Task
 from app.models.workload_snapshot import WorkloadSnapshot
+from app.models.comment import Comment
+from app.models.mention import Mention
+from app.models.notification import Notification
+from app.models.blocker import Blocker
 
-__all__ = ["User", "Role", "Department", "Space", "Project", "TaskStatus", "Task", "WorkloadSnapshot"]
+__all__ = [
+    "User", "Role", "Department", "Space", "Project", "TaskStatus", "Task", "WorkloadSnapshot",
+    "Comment", "Mention", "Notification", "Blocker"
+]

backend/app/models/blocker.py

@@ -0,0 +1,23 @@
+import uuid
+from sqlalchemy import Column, String, Text, DateTime, ForeignKey
+from sqlalchemy.sql import func
+from sqlalchemy.orm import relationship
+from app.core.database import Base
+
+
+class Blocker(Base):
+    __tablename__ = "pjctrl_blockers"
+
+    id = Column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    task_id = Column(String(36), ForeignKey("pjctrl_tasks.id", ondelete="CASCADE"), nullable=False)
+    reported_by = Column(String(36), ForeignKey("pjctrl_users.id"), nullable=False)
+    reason = Column(Text, nullable=False)
+    resolved_by = Column(String(36), ForeignKey("pjctrl_users.id"), nullable=True)
+    resolution_note = Column(Text, nullable=True)
+    created_at = Column(DateTime, server_default=func.now(), nullable=False)
+    resolved_at = Column(DateTime, nullable=True)
+
+    # Relationships
+    task = relationship("Task", back_populates="blockers")
+    reporter = relationship("User", foreign_keys=[reported_by], back_populates="reported_blockers")
+    resolver = relationship("User", foreign_keys=[resolved_by], back_populates="resolved_blockers")

backend/app/models/comment.py

@@ -0,0 +1,26 @@
+import uuid
+from sqlalchemy import Column, String, Text, Boolean, DateTime, ForeignKey
+from sqlalchemy.sql import func
+from sqlalchemy.orm import relationship
+from app.core.database import Base
+
+
+class Comment(Base):
+    __tablename__ = "pjctrl_comments"
+
+    id = Column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    task_id = Column(String(36), ForeignKey("pjctrl_tasks.id", ondelete="CASCADE"), nullable=False)
+    parent_comment_id = Column(String(36), ForeignKey("pjctrl_comments.id", ondelete="CASCADE"), nullable=True)
+    author_id = Column(String(36), ForeignKey("pjctrl_users.id"), nullable=False)
+    content = Column(Text, nullable=False)
+    is_edited = Column(Boolean, default=False, nullable=False)
+    is_deleted = Column(Boolean, default=False, 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
+    task = relationship("Task", back_populates="comments")
+    author = relationship("User", back_populates="comments")
+    parent_comment = relationship("Comment", remote_side=[id], back_populates="replies")
+    replies = relationship("Comment", back_populates="parent_comment", cascade="all, delete-orphan")
+    mentions = relationship("Mention", back_populates="comment", cascade="all, delete-orphan")

backend/app/models/mention.py

@@ -0,0 +1,18 @@
+import uuid
+from sqlalchemy import Column, String, DateTime, ForeignKey
+from sqlalchemy.sql import func
+from sqlalchemy.orm import relationship
+from app.core.database import Base
+
+
+class Mention(Base):
+    __tablename__ = "pjctrl_mentions"
+
+    id = Column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    comment_id = Column(String(36), ForeignKey("pjctrl_comments.id", ondelete="CASCADE"), nullable=False)
+    mentioned_user_id = Column(String(36), ForeignKey("pjctrl_users.id"), nullable=False)
+    created_at = Column(DateTime, server_default=func.now(), nullable=False)
+
+    # Relationships
+    comment = relationship("Comment", back_populates="mentions")
+    mentioned_user = relationship("User", back_populates="mentions")

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")

backend/app/models/task.py

@@ -43,3 +43,7 @@ class Task(Base):
     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")
+
+    # Collaboration relationships
+    comments = relationship("Comment", back_populates="task", cascade="all, delete-orphan")
+    blockers = relationship("Blocker", back_populates="task", cascade="all, delete-orphan")

backend/app/models/user.py

@@ -29,3 +29,10 @@ class User(Base):
     owned_projects = relationship("Project", foreign_keys="Project.owner_id", back_populates="owner")
     assigned_tasks = relationship("Task", foreign_keys="Task.assignee_id", back_populates="assignee")
     created_tasks = relationship("Task", foreign_keys="Task.created_by", back_populates="creator")
+
+    # Collaboration relationships
+    comments = relationship("Comment", back_populates="author")
+    mentions = relationship("Mention", back_populates="mentioned_user")
+    notifications = relationship("Notification", back_populates="user", cascade="all, delete-orphan")
+    reported_blockers = relationship("Blocker", foreign_keys="Blocker.reported_by", back_populates="reporter")
+    resolved_blockers = relationship("Blocker", foreign_keys="Blocker.resolved_by", back_populates="resolver")