fix: count a run completed at the DONE transition, not after it (researchx#28)

The cost counter was incremented by a watcher that awaited the run task, so it
lagged the status flip: the run read as done while /api/config still counted
zero completed runs. The increment now happens in the same step as the
transition to DONE, so any client that observes the run as done also observes
the counter.

Refs researchx#28

backend/src/researchx/api/app.py

@@ -226,6 +226,7 @@ def create_app(
             storage=storage,
             registry=registry,
             event_bus=bus,
+            on_complete=runs.note_completed,
         )
         probe_data: dict[str, Any] | None = None
         if supports_probe(model):
@@ -240,9 +241,7 @@ def create_app(
         runs.add(run_id, actor, task, bus)
 
         async def _watch() -> None:
-            state = await task
-            if state.state == RunStateName.DONE:
-                runs.note_completed()
+            await task
 
         asyncio.create_task(_watch())
         return _summary(run_id)

backend/src/researchx/engine/actor.py

@@ -1,4 +1,5 @@
 import asyncio
+from collections.abc import Callable
 from datetime import UTC, datetime
 from typing import Any
 
@@ -34,6 +35,7 @@ class RunActor:
         storage: LocalStorageAdapter,
         registry: MethodologyRegistry,
         event_bus: EventBus,
+        on_complete: Callable[[], None] | None = None,
     ) -> None:
         self._config = run_config
         self._model = model
@@ -41,6 +43,7 @@ class RunActor:
         self._storage = storage
         self._registry = registry
         self._bus = event_bus
+        self._on_complete = on_complete
         self._store = EvidenceStore()
         self.state = RunState(
             run_id=run_config.run_id,
@@ -83,6 +86,8 @@ class RunActor:
     async def _checkpoint(self, target: RunStateName) -> None:
         if self.state.state != target:
             self.state.transition(target)
+            if target is RunStateName.DONE and self._on_complete is not None:
+                self._on_complete()
         await self._persist()
 
     async def _wait_if_paused(self) -> None: