CUST-WP-0074-T02: task wait qualifiers in the hub read model

Adds tasks.decision_id and workplan_dependencies.from_task_id (migration
e8f9a0b1c2d3). A from_task_id column is needed rather than
from_workplan_id plus description: the partial unique indexes key on
(from_workplan_id, target, relationship_type), so two tasks in one
workplan waiting on the same target could not both be indexed, and the
derived wait_kind needs per-task attribution. The two existing unique
indexes are narrowed to frontmatter edges (from_task_id IS NULL) and two
task-origin counterparts are added; downgrade deletes task-origin rows
before restoring the old indexes.

POST /workplans/{id}/dependencies/ accepts from_task_id (must belong to
the from workplan; a task cannot depend on itself). TaskCreate/Update/Read
carry decision_id.

Derived read-model fields, no new write routes (rule 5):
- TaskRead.wait_kind: external | human | both | unqualified, null unless
  status is wait (dependency rows from this task / needs_human).
- WorkplanRead.blocked_kind: human | external | none, null unless status
  is blocked. Human wins over external.
Both come from api/services/wait_kind.py, applied on GET /tasks/,
GET /tasks/{id}, GET /workplans/ and GET /workplans/{id}.

The identifier-migration reference-count proof now includes from_task_id
(FK count guard 22 -> 23).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: sonnet
Assistant-Process: 237582@bnt-lap001
Assistant-Session: f2b3d9f1-8fb9-4b9c-bc2b-837ec5dfc826

migrations/versions/e8f9a0b1c2d3_task_wait_qualifiers.py

@@ -0,0 +1,103 @@
+"""task wait qualifiers (CUST-WP-0074-T02)
+
+Adds ``tasks.decision_id`` (record id of the decision a human-gated wait hangs
+on) and ``workplan_dependencies.from_task_id`` so that task-block
+``depends_on`` edges carry the waiting task as the from side. The two existing
+partial unique indexes are narrowed to frontmatter edges (``from_task_id IS
+NULL``) and two task-origin counterparts are added, so two tasks in one
+workplan may wait on the same target.
+
+Revision ID: e8f9a0b1c2d3
+Revises: d7e8f9a0b1c2
+"""
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects import postgresql
+
+revision = "e8f9a0b1c2d3"
+down_revision = "d7e8f9a0b1c2"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("tasks", sa.Column("decision_id", sa.String(length=120), nullable=True))
+
+    op.add_column(
+        "workplan_dependencies",
+        sa.Column("from_task_id", postgresql.UUID(as_uuid=True), nullable=True),
+    )
+    op.create_foreign_key(
+        "fk_workplan_dependencies_from_task_id",
+        "workplan_dependencies",
+        "tasks",
+        ["from_task_id"],
+        ["id"],
+        ondelete="CASCADE",
+        onupdate="CASCADE",
+    )
+    op.create_index(
+        "ix_workplan_dependencies_from_task_id",
+        "workplan_dependencies",
+        ["from_task_id"],
+    )
+
+    op.drop_index("uq_wp_dep_workplan_target", table_name="workplan_dependencies")
+    op.drop_index("uq_wp_dep_task_target", table_name="workplan_dependencies")
+    op.create_index(
+        "uq_wp_dep_workplan_target",
+        "workplan_dependencies",
+        ["from_workplan_id", "to_workplan_id", "relationship_type"],
+        unique=True,
+        postgresql_where=sa.text("to_workplan_id IS NOT NULL AND from_task_id IS NULL"),
+    )
+    op.create_index(
+        "uq_wp_dep_task_target",
+        "workplan_dependencies",
+        ["from_workplan_id", "to_task_id", "relationship_type"],
+        unique=True,
+        postgresql_where=sa.text("to_task_id IS NOT NULL AND from_task_id IS NULL"),
+    )
+    op.create_index(
+        "uq_wp_dep_from_task_workplan_target",
+        "workplan_dependencies",
+        ["from_task_id", "to_workplan_id", "relationship_type"],
+        unique=True,
+        postgresql_where=sa.text("to_workplan_id IS NOT NULL AND from_task_id IS NOT NULL"),
+    )
+    op.create_index(
+        "uq_wp_dep_from_task_task_target",
+        "workplan_dependencies",
+        ["from_task_id", "to_task_id", "relationship_type"],
+        unique=True,
+        postgresql_where=sa.text("to_task_id IS NOT NULL AND from_task_id IS NOT NULL"),
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("uq_wp_dep_from_task_task_target", table_name="workplan_dependencies")
+    op.drop_index("uq_wp_dep_from_task_workplan_target", table_name="workplan_dependencies")
+    op.drop_index("uq_wp_dep_task_target", table_name="workplan_dependencies")
+    op.drop_index("uq_wp_dep_workplan_target", table_name="workplan_dependencies")
+    # Task-origin rows would collide under the frontmatter-only indexes.
+    op.execute("DELETE FROM workplan_dependencies WHERE from_task_id IS NOT NULL")
+    op.create_index(
+        "uq_wp_dep_workplan_target",
+        "workplan_dependencies",
+        ["from_workplan_id", "to_workplan_id", "relationship_type"],
+        unique=True,
+        postgresql_where=sa.text("to_workplan_id IS NOT NULL"),
+    )
+    op.create_index(
+        "uq_wp_dep_task_target",
+        "workplan_dependencies",
+        ["from_workplan_id", "to_task_id", "relationship_type"],
+        unique=True,
+        postgresql_where=sa.text("to_task_id IS NOT NULL"),
+    )
+    op.drop_index("ix_workplan_dependencies_from_task_id", table_name="workplan_dependencies")
+    op.drop_constraint(
+        "fk_workplan_dependencies_from_task_id", "workplan_dependencies", type_="foreignkey"
+    )
+    op.drop_column("workplan_dependencies", "from_task_id")
+    op.drop_column("tasks", "decision_id")