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/alembic/README

@@ -0,0 +1,27 @@
+Alembic Migrations for Orchard
+
+This directory contains database migration scripts managed by Alembic.
+
+Common Commands:
+  # Generate a new migration (autogenerate from model changes)
+  alembic revision --autogenerate -m "description of changes"
+
+  # Apply all pending migrations
+  alembic upgrade head
+
+  # Rollback one migration
+  alembic downgrade -1
+
+  # Show current migration status
+  alembic current
+
+  # Show migration history
+  alembic history
+
+  # Generate SQL without applying (for review)
+  alembic upgrade head --sql
+
+Notes:
+- Always review autogenerated migrations before applying
+- Test migrations in development before applying to production
+- Migrations are stored in the versions/ directory