Synchronize Stage 12 observability column

[anth-volk]

Fixes #697

Summary

• add the nullable, 36-character observability identifier to the typed Stage 12 report record
• synchronize the DML-only SQLAlchemy mapping with the API-owned table
• exercise the nullable column during infrastructure validation
• add focused contract, mapping, and persistence coverage

This PR contains no Alembic migration, DDL, or schema-changing runtime call. PolicyEngine/policyengine-api remains the sole owner of the database schema.

Verification

• contract tests: 104 passed
• persistence tests: 12 passed
• executor tests: 594 passed, 24 skipped, 2 deselected
• entrypoint tests: 147 passed
• Ruff formatting and lint checks passed for every changed file

A direct per-package Pyright invocation could not resolve dependencies from the existing local package environments. The repository CI static-analysis job remains the canonical check.