Add baseline Grafana observability and stage timing telemetry

[anth-volk]

Fixes #438

Summary

• add baseline OTLP/Grafana-capable observability in shadow mode for the simulation gateway and worker
• instrument gateway routing and worker lifecycle with coarse logs, traces, and metrics
• add stage-level timing telemetry for routing, queue delay, worker phases, completion, and failure paths

Testing

• uv run ruff check projects/policyengine-api-simulation/src/modal/gateway/endpoints.py projects/policyengine-api-simulation/src/modal/observability.py projects/policyengine-api-simulation/src/modal/app.py projects/policyengine-api-simulation/src/modal/simulation.py projects/policyengine-api-simulation/tests/gateway/test_endpoints.py projects/policyengine-api-simulation/tests/test_modal_observability.py libs/policyengine-fastapi/src/policyengine_fastapi/observability/config.py
• uv run pytest projects/policyengine-api-simulation/tests/gateway/test_endpoints.py projects/policyengine-api-simulation/tests/gateway/test_models.py projects/policyengine-api-simulation/tests/test_modal_observability.py
• uv run --extra test pytest libs/policyengine-fastapi/tests/test_observability.py