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/repositories/__init__.py

@@ -0,0 +1,22 @@
+"""
+Repository pattern implementation for data access layer.
+
+Repositories abstract database operations from business logic,
+providing clean interfaces for CRUD operations on each entity.
+"""
+
+from .base import BaseRepository
+from .project import ProjectRepository
+from .package import PackageRepository
+from .artifact import ArtifactRepository
+from .tag import TagRepository
+from .upload import UploadRepository
+
+__all__ = [
+    "BaseRepository",
+    "ProjectRepository",
+    "PackageRepository",
+    "ArtifactRepository",
+    "TagRepository",
+    "UploadRepository",
+]