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()

apps/api/app/db.py

@@ -0,0 +1,17 @@
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker, Session
+
+from app.core.database import get_database_settings
+
+settings = get_database_settings()
+engine = create_engine(settings.database_url)
+SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+
+
+def get_db() -> Session:
+    """Dependency for database session."""
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()