Expose ASB Postgres pool metrics

[haasonsaas]

Summary

• expose Postgres pool gauges on ASB's /metrics endpoint through service-runtime/observability.RegisterDBStats
• carry runtime DB stats out of bootstrap so the API process can register dependency metrics without reaching into store internals
• document and test the new database metrics output

Testing

• go test ./cmd/asb-api ./internal/bootstrap ./internal/api/httpapi
• go test ./...
• git diff --check

Progresses #5

[cursor]

PR Summary

Low Risk
Low risk observability change that adds Prometheus gauges and threads DB pool stats through bootstrap; main risk is metric registration conflicts or nil wiring when Postgres is disabled.

Overview
Adds Postgres connection-pool gauges to ASB’s /metrics endpoint when ASB_POSTGRES_DSN is configured, via service-runtime/observability.RegisterDBStats.

internal/bootstrap.ServiceRuntime now carries a DBStats function (sourced from pgxpool.Stat() in newRepository), and cmd/asb-api registers these runtime metrics at startup; tests and README are updated to validate/document the new metric output.

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