feat: add scratchpad field to session model and schemas

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/schemas/session.py

@@ -1,7 +1,7 @@
 from datetime import datetime
 from typing import Optional, Any
 from uuid import UUID
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, validator
 
 
 class DecisionRecord(BaseModel):
@@ -27,6 +27,7 @@ class SessionUpdate(BaseModel):
     custom_steps: Optional[list[dict[str, Any]]] = None
     ticket_number: Optional[str] = Field(None, max_length=100)
     client_name: Optional[str] = Field(None, max_length=255)
+    scratchpad: Optional[str] = None
 
 
 class SessionResponse(BaseModel):
@@ -42,6 +43,11 @@ class SessionResponse(BaseModel):
     ticket_number: Optional[str] = None
     client_name: Optional[str] = None
     exported: bool
+    scratchpad: str = ""
+
+    @validator('scratchpad', pre=True, always=True)
+    def normalize_scratchpad(cls, v):
+        return v or ""
 
     class Config:
         from_attributes = True
@@ -51,3 +57,7 @@ class SessionExport(BaseModel):
     format: str = Field(default="markdown", pattern="^(text|markdown|html)$")
     include_timestamps: bool = True
     include_tree_info: bool = True
+
+
+class ScratchpadUpdate(BaseModel):
+    scratchpad: str