- Add Database struct with GORM PostgreSQL connection
- Add IMSIAllocation model for IMSI range tracking and allocation
- Add NewDatabase function with connection pooling (10 idle, 100 max connections, 1h lifetime)
- Add AutoMigrate for Subscriber, ServicePlan, Session, UsageRecord, and IMSIAllocation models
- Add InitializeIMSIAllocation with default IMSI range (1-999999999)
- Add CRUD operations for subscribers (Create, Get, GetByIMSI, Update, List)
- Add ListSubscribers with pagination, filtering by status/
