Add baseline Grafana observability and stage timing telemetry

[anth-volk]

Fixes #3410

Summary

• add baseline OTLP/Grafana-capable observability in shadow mode for legacy simulation submission
• instrument the Modal submission client and request lifecycle with coarse logs, traces, and metrics
• add stage-level timing telemetry for setup, submission, and failure handling

Testing

• uv run ruff check policyengine_api/observability.py policyengine_api/libs/simulation_api_modal.py policyengine_api/services/economy_service.py tests/unit/test_observability.py tests/unit/libs/test_simulation_api_modal.py tests/fixtures/libs/simulation_api_modal.py
• uv run python -c "from policyengine_api.observability import get_observability, build_lifecycle_event; obs = get_observability('policyengine-api'); print(type(obs).name); print(build_lifecycle_event(stage='job.setup', status='ok', service='policyengine-api', telemetry={'run_id':'r1'})['stage'])"