Synchronize Stage 12 report mapping with observability column

[anth-volk]

Problem

The API-owned Stage 12 report table now contains the nullable observability_id column added by PolicyEngine/policyengine-api#3852, but the simulation service synchronized SQLAlchemy mapping does not. Deployment infrastructure validation therefore rejects the live table before Stage 12 deployment.

Scope

• add the nullable, length-bounded field to the typed Stage 12 report record;
• add the matching declarative SQLAlchemy column without emitting DDL;
• exercise the column in the infrastructure data-operation check;
• add focused contract and persistence tests.

The API repository remains the sole owner of database migrations.