Add dependency-aware readiness reporting for Postgres, MQTT, auth, and metadata reconcile

[jillesvangurp]

Summary

PR #35 added OTLP telemetry, integration coverage, and local SigNoz wiring, but issue #1 still lacks an operator-facing readiness contract that distinguishes startup success from dependency degradation.

Scope

• define a readiness surface beyond the current /healthz liveness check
• report dependency-specific readiness for Postgres, MQTT, auth/JWKS initialization, and metadata reconcile health
• make startup and runtime unhealthy states visible without requiring trace/log inspection
• document the readiness semantics in the runtime/configuration docs

Why

This is the remaining implementation slice from issue #1 after the observability baseline landed in PR #35.

Part of #1
Related to #35