Add .env-example file and update configuration for environment variables

- Created .env-example for environment variable setup
- Updated README to reflect changes in .env file creation
- Modified config.py to include new environment variables and DATABASE_URL property
- Enhanced database.py with connect and disconnect functions
- Updated main.py to manage database connection lifecycle
- Adjusted compose.yaml for consistent environment variable usage

app/main.py

@@ -1,14 +1,15 @@
 from contextlib import asynccontextmanager
 from fastapi import FastAPI
-from app.database import engine
+from app.database import connect, disconnect
 from app.routers import authors, books
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
-    yield # tables already created by Create.sql
-    await engine.dispose()
+    await connect()
+    yield
+    await disconnect()
 
 app = FastAPI(title="Database API", lifespan=lifespan)
 
-app.include_router(authors.router)
-app.include_router(books.router)
+for router in [authors.router, books.router]:
+    app.include_router(router)