BE-PR-007: refactor Proposal Accept to generate story tasks from all Essentials

- Removed old logic that created a single story/feature task on accept
- Accept now iterates all Essentials under the Proposal
- Each Essential.type maps to a story/* task (feature/improvement/refactor)
- All tasks created in a single transaction
- Added ProposalAcceptResponse and GeneratedTaskSummary schemas
- Proposal must have at least one Essential to be accepted
- No longer writes to deprecated feat_task_id field

app/schemas/schemas.py

@@ -353,6 +353,27 @@ class ProposalDetailResponse(ProposalResponse):
         from_attributes = True
 
 
+class GeneratedTaskSummary(BaseModel):
+    """Brief summary of a task generated from a Proposal Essential."""
+    task_id: int
+    task_code: str
+    task_type: str
+    task_subtype: str
+    title: str
+    essential_id: int
+    essential_code: str
+
+
+class ProposalAcceptResponse(ProposalResponse):
+    """Response for Proposal Accept — includes the generated story tasks."""
+
+    essentials: List[EssentialResponse] = []
+    generated_tasks: List[GeneratedTaskSummary] = []
+
+    class Config:
+        from_attributes = True
+
+
 # Backward-compatible aliases
 ProposeStatusEnum = ProposalStatusEnum
 ProposeBase = ProposalBase