feat: add task blocker dependency graph + actionable now view

- New task_blockers join table (many-to-many, cross-project)
- Cycle detection prevents circular dependencies
- GET /api/tasks/{id}/blockers and /blocking endpoints
- POST/DELETE /api/tasks/{id}/blockers/{blocker_id}
- GET /api/actionable — all non-done tasks with no incomplete blockers
- BlockerPanel.jsx — search & manage blockers per task (via task menu)
- ActionableView.jsx — "what can I do right now?" dashboard grouped by project
- "Now" button in nav header routes to actionable view
- migrate_add_blockers.py migration script for existing databases

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models.py

@@ -1,42 +1,61 @@
-from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey, JSON, Boolean
-from sqlalchemy.orm import relationship
-from datetime import datetime
-from .database import Base
-
-
-# Default statuses for new projects
-DEFAULT_STATUSES = ["backlog", "in_progress", "on_hold", "done"]
-
-
-class Project(Base):
-    __tablename__ = "projects"
-
-    id = Column(Integer, primary_key=True, index=True)
-    name = Column(String(255), nullable=False)
-    description = Column(Text, nullable=True)
-    statuses = Column(JSON, nullable=False, default=DEFAULT_STATUSES)
-    is_archived = Column(Boolean, default=False, nullable=False)
-    created_at = Column(DateTime, default=datetime.utcnow)
-    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
-
-    tasks = relationship("Task", back_populates="project", cascade="all, delete-orphan")
-
-
-class Task(Base):
-    __tablename__ = "tasks"
-
-    id = Column(Integer, primary_key=True, index=True)
-    project_id = Column(Integer, ForeignKey("projects.id"), nullable=False)
-    parent_task_id = Column(Integer, ForeignKey("tasks.id"), nullable=True)
-    title = Column(String(500), nullable=False)
-    description = Column(Text, nullable=True)
-    status = Column(String(50), default="backlog", nullable=False)
-    sort_order = Column(Integer, default=0)
-    estimated_minutes = Column(Integer, nullable=True)
-    tags = Column(JSON, nullable=True)
-    flag_color = Column(String(50), nullable=True)
-    created_at = Column(DateTime, default=datetime.utcnow)
-    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
-
-    project = relationship("Project", back_populates="tasks")
-    parent = relationship("Task", remote_side=[id], backref="subtasks")
+from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey, JSON, Boolean, Table
+from sqlalchemy.orm import relationship
+from datetime import datetime
+from .database import Base
+
+
+# Default statuses for new projects
+DEFAULT_STATUSES = ["backlog", "in_progress", "on_hold", "done"]
+
+
+# Association table for task blocker relationships (many-to-many)
+task_blockers = Table(
+    "task_blockers",
+    Base.metadata,
+    Column("task_id", Integer, ForeignKey("tasks.id", ondelete="CASCADE"), primary_key=True),
+    Column("blocked_by_id", Integer, ForeignKey("tasks.id", ondelete="CASCADE"), primary_key=True),
+)
+
+
+class Project(Base):
+    __tablename__ = "projects"
+
+    id = Column(Integer, primary_key=True, index=True)
+    name = Column(String(255), nullable=False)
+    description = Column(Text, nullable=True)
+    statuses = Column(JSON, nullable=False, default=DEFAULT_STATUSES)
+    is_archived = Column(Boolean, default=False, nullable=False)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+
+    tasks = relationship("Task", back_populates="project", cascade="all, delete-orphan")
+
+
+class Task(Base):
+    __tablename__ = "tasks"
+
+    id = Column(Integer, primary_key=True, index=True)
+    project_id = Column(Integer, ForeignKey("projects.id"), nullable=False)
+    parent_task_id = Column(Integer, ForeignKey("tasks.id"), nullable=True)
+    title = Column(String(500), nullable=False)
+    description = Column(Text, nullable=True)
+    status = Column(String(50), default="backlog", nullable=False)
+    sort_order = Column(Integer, default=0)
+    estimated_minutes = Column(Integer, nullable=True)
+    tags = Column(JSON, nullable=True)
+    flag_color = Column(String(50), nullable=True)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+
+    project = relationship("Project", back_populates="tasks")
+    parent = relationship("Task", remote_side=[id], backref="subtasks")
+
+    # blockers: tasks that must be done before this task can start
+    # blocking: tasks that this task is holding up
+    blockers = relationship(
+        "Task",
+        secondary=task_blockers,
+        primaryjoin=lambda: Task.id == task_blockers.c.task_id,
+        secondaryjoin=lambda: Task.id == task_blockers.c.blocked_by_id,
+        backref="blocking",
+    )