Fix shutdown-order bug: close source clients after workers stop

Workers share the same Source instances as pollers and use them for
fetch(). PollerManager.stop() was closing httpx clients before the
worker pool drained, so in-flight fetches during the shutdown grace
hit a closed client.

- Move source closing out of stop() into a separate close_sources()
- Call close_sources() after _stop_pool() in both serve() and
  run_batch()
- Promote aclose() to the Source protocol with no-op defaults for
  FS and S3, removing the hasattr duck-typing

haiku_rag_slim/haiku/rag/ingester/app.py

@@ -181,6 +181,7 @@ class IngesterApp:
                     await asyncio.gather(api_task, return_exceptions=True)
                 await self._pollers.stop()
                 await self._stop_pool()
+                await self._pollers.close_sources()
 
     async def run_batch(self) -> BatchReport:
         """Run one discover() sweep across every configured source, drain the
@@ -213,6 +214,7 @@ class IngesterApp:
                 )
             finally:
                 await self._stop_pool()
+                await self._pollers.close_sources()
 
     async def _maybe_start_api(self, api: bool):
         """Spin up the FastAPI control plane on an asyncio task. Returns

haiku_rag_slim/haiku/rag/ingester/pollers/manager.py

@@ -96,9 +96,13 @@ class PollerManager:
             await asyncio.gather(*self._tasks, return_exceptions=True)
         self._tasks.clear()
         self._started = False
+
+    async def close_sources(self) -> None:
+        """Close all source adapters (e.g. HTTP connection pools). Must be
+        called after the worker pool has fully stopped so in-flight fetches
+        don't hit a closed client."""
         for source in self.sources:
-            if hasattr(source, "aclose"):
-                await source.aclose()
+            await source.aclose()
 
     @property
     def pollers(self) -> list[BasePoller]:

haiku_rag_slim/haiku/rag/ingester/sources/base.py

@@ -61,6 +61,11 @@ class Source(Protocol):
         """
         ...
 
+    async def aclose(self) -> None:
+        """Release any resources held by the source (e.g. HTTP connection
+        pools). Called once during shutdown, after all workers have stopped."""
+        ...
+
     async def fetch(self, uri: str) -> FetchResult: ...
 
     def discover(

haiku_rag_slim/haiku/rag/ingester/sources/fs.py

@@ -75,6 +75,9 @@ class FSSource:
             return False
         return self._resolve_within_root(uri) is not None
 
+    async def aclose(self) -> None:  # pragma: no cover - no resources to release
+        pass
+
     async def head(self, uri: str) -> str | None:
         path = self._resolve_within_root(uri)
         if path is None or not path.exists():

haiku_rag_slim/haiku/rag/ingester/sources/s3.py

@@ -63,6 +63,9 @@ class S3Source:
     def supports(self, uri: str) -> bool:
         return uri.startswith(self.uri_prefix)
 
+    async def aclose(self) -> None:  # pragma: no cover - no resources to release
+        pass
+
     async def head(self, uri: str) -> str | None:
         import obstore  # type: ignore[import-not-found]
 

tests/ingester/test_sources_base.py

@@ -59,6 +59,9 @@ def test_source_protocol_runtime_checkable():
         def supports(self, uri: str) -> bool:
             return True
 
+        async def aclose(self) -> None:
+            pass
+
         async def head(self, uri: str):
             return None