feat(03-01): Zone service, API routes, and 25 integration tests
- zone-service.ts: createZone, updateZone, listZones, getZone,
assignSubscriberToZone, removeSubscriberFromZone, assignCollectorToZone,
removeCollectorFromZone, getCollectorZones, getCollectorSubscribers
- API routes: GET/POST /api/zones, GET/PUT /api/zones/[id],
POST/DELETE /api/zones/[id]/subscribers, GET /api/collectors/[id]/subscribers
- 25 integration tests: zone CRUD, subscriber assignment, collector scoping,
no-zone-assignment security boundary, cross-tenant isolation
- Fix subscriber-service.ts: zone String? -> zoneId FK (deviation Rule 1)
- Fix subscriber API routes: zone -> zoneId (deviation Rule 1)
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