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/packages/harness/deerflow/config/run_events_config.py

@@ -0,0 +1,33 @@
+"""Run event storage configuration.
+
+Controls where run events (messages + execution traces) are persisted.
+
+Backends:
+- memory: In-memory storage, data lost on restart. Suitable for
+  development and testing.
+- db: SQL database via SQLAlchemy ORM. Provides full query capability.
+  Suitable for production deployments.
+- jsonl: Append-only JSONL files. Lightweight alternative for
+  single-node deployments that need persistence without a database.
+"""
+
+from __future__ import annotations
+
+from typing import Literal
+
+from pydantic import BaseModel, Field
+
+
+class RunEventsConfig(BaseModel):
+    backend: Literal["memory", "db", "jsonl"] = Field(
+        default="memory",
+        description="Storage backend for run events. 'memory' for development (no persistence), 'db' for production (SQL queries), 'jsonl' for lightweight single-node persistence.",
+    )
+    max_trace_content: int = Field(
+        default=10240,
+        description="Maximum trace content size in bytes before truncation (db backend only).",
+    )
+    track_token_usage: bool = Field(
+        default=True,
+        description="Whether RunJournal should accumulate token counts to RunRow.",
+    )