feat: add backend validation for fallback steps (Task 16)

Validate fallback_steps in procedural flow validation: required fields,
no nested fallback_steps, no duplicate IDs. Add FallbackStepRecord schema
and fallback_decisions field to SessionResponse.

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

backend/app/schemas/session.py

@@ -98,6 +98,9 @@ class SessionResponse(BaseModel):
     psa_ticket_id: Optional[str] = None
     psa_connection_id: Optional[UUID] = None
 
+    # Fallback step decisions
+    fallback_decisions: list[dict[str, Any]] = Field(default_factory=list)
+
     class Config:
         from_attributes = True
 
@@ -123,6 +126,14 @@ class SessionComplete(BaseModel):
     next_steps: Optional[str] = None
 
 
+class FallbackStepRecord(BaseModel):
+    parent_step_id: str
+    fallback_step_id: str
+    completed_at: str | None = None
+    notes: str | None = None
+    outcome: Literal['resolved', 'not_resolved', 'skipped']
+
+
 class SessionVariablesUpdate(BaseModel):
     """Partial update to session variables (dict merge)."""
     variables: dict[str, str] = Field(..., description="Key-value pairs to merge into session_variables")