feat(dunning): add billing dunning module — campaigns, policies, retries
- types.ts: DunningCampaignDoc, DunningPolicyDoc + 3 Zod schemas + helpers
- repository.ts: campaign CRUD, policy management, retry scheduling
- routes.ts: 11 endpoints (campaign lifecycle, policy CRUD, retry, grace period)
- dunning.test.ts: 20 schema + helper tests (getNextRetryTime, isGracePeriodExpired)
- Configurable retry schedules, grace periods, auto-downgrade
- Campaign status lifecycle: active → resolved/escalated
- Cosmos containers: dunning_campaigns, dunning_policies