feat: billing — usage tracking per SQS operation in PostgreSQL

- New package app/billing: Init/RecordUsage/Close with auto-migrate
- Integration in actionHandler: record tenant_id, operation, msg_count, msg_bytes
- Helm chart: billing section in values.yaml, secret-billing.yaml, env vars in deployment
- Optional: billing disabled by default (BILLING_PG_HOST not set = no-op)
- Table: sqs_usage_records with index on (tenant_id, recorded_at)

deployments/helm/shared-sqs/templates/secret-billing.yaml

@@ -0,0 +1,15 @@
+# secret-billing.yaml — Secret с PostgreSQL credentials для billing
+# Создаётся только если billing.enabled=true и existingSecret не указан
+# Created: 2026-04-12
+{{- if and .Values.billing.enabled (not .Values.billing.postgres.existingSecret) }}
+apiVersion: v1
+kind: Secret
+metadata:
+  name: shared-sqs-billing-pg
+  namespace: {{ .Values.namespace }}
+  labels:
+    {{- include "shared-sqs.labels" . | nindent 4 }}
+type: Opaque
+stringData:
+  password: {{ required "billing.postgres.password is required when billing is enabled" .Values.billing.postgres.password | quote }}
+{{- end }}