test: run the README cleanup example against PostgreSQL

tests/test_readme_examples.py

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+import dataclasses
+import datetime as dt
+
+from db_retry import Transaction, postgres_retry
+from sqlalchemy import func, insert, select
+from sqlalchemy.ext.asyncio import AsyncEngine, AsyncSession, async_sessionmaker
+
+from sqlalchemy_inbox import Inbox
+from tests.conftest import Tables
+
+
+@dataclasses.dataclass(kw_only=True, frozen=True, slots=True)
+class CleanupInboxUseCase:
+    transaction: Transaction
+    inbox: Inbox
+
+    async def __call__(self, *, retention: dt.timedelta, batch_size: int) -> None:
+        async with self.transaction:
+            cutoff = await self.inbox.cleanup_cutoff(retention=retention)
+        while await self._delete_batch(cutoff=cutoff, limit=batch_size):
+            pass
+
+    @postgres_retry
+    async def _delete_batch(self, *, cutoff: dt.datetime, limit: int) -> int:
+        async with self.transaction:
+            deleted = await self.inbox.delete_expired_batch(cutoff=cutoff, limit=limit)
+            await self.transaction.commit()
+        return deleted
+
+
+async def test_readme_cleanup_use_case_deletes_every_expired_marker(
+    pg_engine: AsyncEngine, tables: Tables, session_maker: async_sessionmaker[AsyncSession]
+) -> None:
+    async with pg_engine.begin() as conn:
+        for index in range(5):
+            await conn.execute(
+                insert(tables.inbox).values(
+                    scope="s", message_id=f"old-{index}", processed_at=func.now() - dt.timedelta(hours=2)
+                )
+            )
+        await conn.execute(insert(tables.inbox).values(scope="s", message_id="recent"))
+
+    session = session_maker()
+    use_case = CleanupInboxUseCase(transaction=Transaction(session=session), inbox=Inbox(session, tables.inbox))
+    await use_case(retention=dt.timedelta(hours=1), batch_size=2)
+
+    async with pg_engine.connect() as conn:
+        remaining = list((await conn.execute(select(tables.inbox.c.message_id))).scalars())
+    assert remaining == ["recent"]