Add explicit migration operator guidance and surface on schema mismatch

[DeepZone]

Motivation

• Provide a single, clear operator guidance message for database migration mismatches and surface it both in system status and runtime schema-mismatch errors so operators have exact commands to remediate.

Description

• Add MIGRATION_OPERATOR_GUIDANCE to backend/app/core/system_status.py and use it in build_system_status when migration_status is not up_to_date.
• Replace the inline 503 error text in backend/app/api/routes_checks.py OperationalError handling with the shared MIGRATION_OPERATOR_GUIDANCE so persistence failures due to missing migration columns return consistent remediation steps.

Testing

• Ran backend tests cd backend && pytest -q (107 passed) and frontend build cd frontend && npm run build (successful), and executed Alembic/app smoke checks with PYTHONPATH=. (alembic heads, alembic current, and python -c "from app.main import app; print('app import ok', bool(app))") which succeeded, noting alembic current fails in this shell without PYTHONPATH=..

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Codex Task