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/proposal.py

@@ -78,6 +78,14 @@ class Proposal(Base):
         lazy="select",
     )
 
+    # BE-PR-008: reverse lookup — story tasks generated from this Proposal
+    generated_tasks = relationship(
+        "Task",
+        foreign_keys="Task.source_proposal_id",
+        lazy="select",
+        viewonly=True,
+    )
+
     # ---- convenience alias ------------------------------------------------
     @hybrid_property
     def proposal_code(self) -> str | None: