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)

go.sum

@@ -1,3 +1,7 @@
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@@ -31,6 +39,8 @@ github.com/sirupsen/logrus v1.9.0/go.mod h1:naHLuLoDiP4jHNo9R0sCBMtWGeIprob74mVs
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