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/config.py

@@ -18,6 +18,12 @@ class Settings(BaseSettings):
     database_dbname: str = "orchard"
     database_sslmode: str = "disable"
 
+    # Database connection pool settings
+    database_pool_size: int = 5  # Number of connections to keep open
+    database_max_overflow: int = 10  # Max additional connections beyond pool_size
+    database_pool_timeout: int = 30  # Seconds to wait for a connection from pool
+    database_pool_recycle: int = 1800  # Recycle connections after this many seconds (30 min)
+
     # S3
     s3_endpoint: str = ""
     s3_region: str = "us-east-1"