Let teardown and activate be asked for one flow

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01StpRc2C6au1WJ1EUU7fsfu

backend/fluksio/flow/controller.py

@@ -510,12 +510,14 @@ class FlowController:
             }
         )
 
-    async def _teardown(self) -> None:
-        """Stop everything the previous pipeline started."""
+    async def _teardown(self, flow: str | None = None) -> None:
+        """Stop everything the previous pipeline started, or one flow's share."""
         for entry in self.loaded.values():
             node = entry.node
             if node is None:
                 continue
+            if flow is not None and entry.flow != flow:
+                continue
             try:
                 await asyncio.wait_for(node.stop(self.app), NODE_STOP_TIMEOUT)
             except asyncio.TimeoutError:
@@ -529,9 +531,12 @@ class FlowController:
             except Exception:
                 logger.exception("Error stopping node '%s'", entry.id)
         # After the nodes, so a loop still winding down is not restarted.
-        await self.supervisor.cancel_all()
+        if flow is None:
+            await self.supervisor.cancel_all()
+        else:
+            await self.supervisor.cancel_flow(flow)
 
-    async def _activate(self) -> None:
+    async def _activate(self, flow: str | None = None) -> None:
         """Start subscriptions, schedules and webhooks of the new pipeline.
 
         What that means per node type is the node's own business — a connector
@@ -541,6 +546,8 @@ class FlowController:
             node = entry.node
             if node is None:
                 continue
+            if flow is not None and entry.flow != flow:
+                continue
             # A stopped flow gets no subscriptions, schedules or webhooks —
             # that is what stopping it means. Nor does a batch flow, which has
             # no outside to listen to.