feat(persistence): add RunEventStore ABC + MemoryRunEventStore

Phase 2-A prerequisite for event storage: adds the unified run event
stream interface (RunEventStore) with an in-memory implementation,
RunEventsConfig, gateway integration, and comprehensive tests (27 cases).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/gateway/deps.py

@@ -14,6 +14,7 @@ from contextlib import AsyncExitStack, asynccontextmanager
 from fastapi import FastAPI, HTTPException, Request
 
 from deerflow.runtime import RunManager, StreamBridge
+from deerflow.runtime.events.store.base import RunEventStore
 from deerflow.runtime.runs.store.base import RunStore
 
 
@@ -46,6 +47,12 @@ async def langgraph_runtime(app: FastAPI) -> AsyncGenerator[None, None]:
         # RunRepository when models are implemented)
         app.state.run_store = MemoryRunStore()
 
+        # Initialize run event store (MemoryRunEventStore for now)
+        # TODO(Phase 2-B): switch to db/jsonl backend based on config.run_events.backend
+        from deerflow.runtime.events.store.memory import MemoryRunEventStore
+
+        app.state.run_event_store = MemoryRunEventStore()
+
         try:
             yield
         finally:
@@ -86,6 +93,14 @@ def get_store(request: Request):
     return getattr(request.app.state, "store", None)
 
 
+def get_run_event_store(request: Request) -> RunEventStore:
+    """Return the RunEventStore, or 503 if not available."""
+    store = getattr(request.app.state, "run_event_store", None)
+    if store is None:
+        raise HTTPException(status_code=503, detail="Run event store not available")
+    return store
+
+
 def get_run_store(request: Request) -> RunStore:
     """Return the RunStore, or 503 if not available."""
     store = getattr(request.app.state, "run_store", None)