feat: add Pydantic schemas for branching, handoffs, and resolution outputs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/schemas/session_handoff.py

@@ -0,0 +1,57 @@
+"""Pydantic schemas for session handoffs."""
+from __future__ import annotations
+from typing import Any
+from uuid import UUID
+from datetime import datetime
+from pydantic import BaseModel, Field
+
+
+class HandoffCreateRequest(BaseModel):
+    intent: str = Field(..., pattern="^(park|escalate)$")
+    engineer_notes: str | None = None
+    priority: str = Field("normal", pattern="^(normal|elevated)$")
+
+
+class HandoffResponse(BaseModel):
+    id: UUID
+    session_id: UUID
+    handed_off_by: UUID
+    intent: str
+    source_branch_id: UUID | None
+    snapshot: dict[str, Any]
+    ai_assessment: str | None
+    ai_assessment_data: dict[str, Any] | None
+    artifacts: list[dict[str, Any]] | None
+    engineer_notes: str | None
+    priority: str
+    claimed_by: UUID | None
+    claimed_at: datetime | None
+    psa_note_pushed: bool
+    notification_sent: bool
+    created_at: datetime
+    model_config = {"from_attributes": True}
+
+
+class HandoffClaimRequest(BaseModel):
+    pass
+
+
+class HandoffBriefingResponse(BaseModel):
+    briefing: str
+    handoff: HandoffResponse
+
+
+class QueueItemResponse(BaseModel):
+    handoff_id: UUID
+    session_id: UUID
+    intent: str
+    problem_summary: str | None
+    problem_domain: str | None
+    priority: str
+    handed_off_by_name: str | None
+    engineer_notes: str | None
+    branch_count: int = 0
+    created_at: datetime
+    claimed_by: UUID | None
+    claimed_at: datetime | None
+    model_config = {"from_attributes": True}