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_branch.py

@@ -0,0 +1,83 @@
+"""Pydantic schemas for session branches and fork points."""
+from __future__ import annotations
+from typing import Any
+from uuid import UUID
+from datetime import datetime
+from pydantic import BaseModel, Field
+
+
+class BranchCreate(BaseModel):
+    label: str = Field(..., max_length=200)
+    status: str = "untried"
+
+
+class BranchUpdate(BaseModel):
+    status: str = Field(..., pattern="^(active|dead_end|solved|untried|revived)$")
+    status_reason: str | None = None
+
+
+class BranchResponse(BaseModel):
+    id: UUID
+    session_id: UUID
+    parent_branch_id: UUID | None
+    fork_point_step_id: UUID | None
+    branch_order: int
+    label: str
+    status: str
+    status_reason: str | None
+    status_changed_at: datetime | None
+    context_summary: dict[str, Any] | None
+    evidence_from_branch_id: UUID | None
+    evidence_description: str | None
+    step_count: int = 0
+    created_at: datetime
+    updated_at: datetime
+    model_config = {"from_attributes": True}
+
+
+class BranchTreeResponse(BaseModel):
+    branches: list[BranchResponse]
+    active_branch_id: UUID | None
+
+
+class ForkOption(BaseModel):
+    label: str = Field(..., max_length=200)
+    description: str = Field(..., max_length=500)
+
+
+class ForkCreateRequest(BaseModel):
+    fork_reason: str = Field(..., min_length=5, max_length=2000)
+    options: list[ForkOption] = Field(..., min_length=2, max_length=10)
+
+
+class ForkPointResponse(BaseModel):
+    id: UUID
+    session_id: UUID
+    parent_branch_id: UUID
+    trigger_step_id: UUID | None
+    fork_reason: str
+    options: list[dict[str, Any]]
+    created_at: datetime
+    model_config = {"from_attributes": True}
+
+
+class BranchSwitchResponse(BaseModel):
+    active_branch_id: UUID
+    branch: BranchResponse
+    conversation_messages: list[dict[str, Any]]
+
+
+class ReviveRequest(BaseModel):
+    evidence_from_branch_id: UUID
+    evidence_description: str = Field(..., min_length=5, max_length=2000)
+
+
+class BranchMessageRequest(BaseModel):
+    message: str = Field(..., min_length=1, max_length=8000)
+    upload_ids: list[UUID] = Field(default_factory=list, max_length=10)
+
+
+class BranchMessageResponse(BaseModel):
+    content: str
+    branch_id: UUID
+    step_id: UUID | None = None