feat: add PostgreSQL persistence layer

- adapters: SQLAlchemy implementations for all 4 repositories (Project, AssetLibrary, Asset, IngestJob)
- models: SQLAlchemy ORM models with proper schema
- database: connection config and session management
- tests: SQLAlchemy repository integration test (in-memory SQLite)
- all 7 integration tests passing

apps/api/app/core/database.py

@@ -0,0 +1,9 @@
+from pydantic import BaseModel
+
+
+class DatabaseSettings(BaseModel):
+    database_url: str = "postgresql://postgres:postgres@postgres:5432/xiaoxia_saas"
+
+
+def get_database_settings() -> DatabaseSettings:
+    return DatabaseSettings()