Implement database storage layer
- Add connection pool configuration (pool_size, max_overflow, timeout, recycle)
- Add transaction management utilities (transaction, savepoint, retry_on_deadlock)
- Create repository pattern classes for all entities (Project, Package, Artifact, Tag, Upload)
- Implement ref_count decrement and cleanup service
- Add query helper functions (search, filtering, pagination, stats)
- Add database constraints (check_ref_count_non_negative, check_size_positive)
- Add performance indexes (idx_artifacts_ref_count, composite indexes for packages/tags)
- Initialize Alembic migrations for future schema changes
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