Add Stage 12 observability identifier schema

[anth-volk]

Fixes #3851

Summary

• add a nullable observability_id column to stage12_evaluation_reports
• add the corresponding SQLModel field
• advance and centralize the expected v2 Alembic revision
• verify the generated migration, downgrade, schema parity, and catalog revision check

This is an additive schema precursor for the API and simulation observability rollout. It does not change request payloads, logging, tracing, or simulation behavior.

Verification

• ruff check passed for all changed Python files
• ruff format --check passed for all changed Python files
• v2 unit suite: 525 tests passed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.48%. Comparing base (d223864) to head (dbb4c12).

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3852       +/-   ##
===========================================
+ Coverage   46.36%   87.48%   +41.11%     
===========================================
  Files         178      193       +15     
  Lines       10419    11538     +1119     
  Branches     1759     2008      +249     
===========================================
+ Hits         4831    10094     +5263     
+ Misses       5199      882     -4317     
- Partials      389      562      +173     

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