BE-PR-008: add Proposal Accept tracking fields (source_proposal_id, source_essential_id)

- Add source_proposal_id and source_essential_id FK columns to Task model
- Populate tracking fields during Proposal Accept task generation
- Add generated_tasks relationship on Proposal model for reverse lookup
- Expose source_proposal_id/source_essential_id in TaskResponse schema
- Add GeneratedTaskBrief schema and include generated_tasks in ProposalDetailResponse
- Proposal detail endpoint now returns generated story tasks with status

app/models/task.py

@@ -37,6 +37,17 @@ class Task(Base):
     assignee_id = Column(Integer, ForeignKey("users.id"), nullable=True)
     created_by_id = Column(Integer, ForeignKey("users.id"), nullable=True)
     
+    # Proposal Accept tracking (BE-PR-008)
+    # When a task is generated from Proposal Accept, these record the source.
+    source_proposal_id = Column(
+        Integer, ForeignKey("proposes.id"), nullable=True,
+        comment="Proposal that generated this task via accept (NULL if manually created)",
+    )
+    source_essential_id = Column(
+        Integer, ForeignKey("essentials.id"), nullable=True,
+        comment="Essential that generated this task via accept (NULL if manually created)",
+    )
+
     # Tags (comma-separated)
     tags = Column(String(500), nullable=True)