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

backend/app/services/__init__.py

@@ -0,0 +1,9 @@
+"""
+Service layer for business logic.
+"""
+
+from .artifact_cleanup import ArtifactCleanupService
+
+__all__ = [
+    "ArtifactCleanupService",
+]