Add Prometheus metrics + vmagent push to obs.88oakapps.com

Adds internal/prom package with histograms for HTTP, GORM, B2, APNs, and
FCM, wired into the Echo router (HTTPMiddleware + /metrics) and GORM via
statement-level callbacks (no ctx plumbing needed). Storage and push
clients call ObserveB2Upload / ObserveAPNsSend / ObserveFCMSend at the
network round-trip points.

Existing internal/monitoring metrics move to /metrics/legacy so the
canonical /metrics emits proper histogram buckets for p50/p95/p99 rollups.

deploy-k3s/manifests/observability/vmagent.yaml deploys a single-replica
vmagent in the honeydue namespace that scrapes api Pods on :8000/metrics
every 15s and remote-writes to https://obs.88oakapps.com/api/v1/write
with a bearer token (substituted at deploy time from OBS_INGEST_TOKEN
in deploy/prod.env). NetworkPolicies allow vmagent egress to api Pods
and to the public obs endpoint over :443; the obs side runs
VictoriaMetrics + Jaeger + Grafana on 88oakappsUpdate.

docs/observability-plan.md captures the full plan including resource
budget, instrumentation table, 4-step rollout, and migration triggers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

go.sum

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