feat(gateway): expose pipeline status diagnostics

[YOMXXX]

Summary

• add read-only GET /status diagnostics for Gateway users
• expose store availability, scheduler state, queue sizes, and per-session L1/L2 timer progress
• add focused pipeline manager tests for waiting L1 status and unknown sessions

Refs #64

Verification

• npm test -- src/utils/pipeline-manager.test.ts
• npm test
• npm run build

[Maxwell-Code07]

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