feat(persistence): add ORM models, repositories, DB/JSONL event stores, RunJournal, and API endpoints

Phase 2-B: run persistence + event storage + token tracking.

- ORM models: RunRow (with token fields), ThreadMetaRow, RunEventRow
- RunRepository implements RunStore ABC via SQLAlchemy ORM
- ThreadMetaRepository with owner access control
- DbRunEventStore with trace content truncation and cursor pagination
- JsonlRunEventStore with per-run files and seq recovery from disk
- RunJournal (BaseCallbackHandler) captures LLM/tool/lifecycle events,
  accumulates token usage by caller type, buffers and flushes to store
- RunManager now accepts optional RunStore for persistent backing
- Worker creates RunJournal, writes human_message, injects callbacks
- Gateway deps use factory functions (RunRepository when DB available)
- New endpoints: messages, run messages, run events, token-usage
- ThreadCreateRequest gains assistant_id field
- 92 tests pass (33 new), zero regressions

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

backend/packages/harness/deerflow/persistence/models/thread_meta.py

@@ -0,0 +1,23 @@
+"""ORM model for thread metadata."""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime
+
+from sqlalchemy import JSON, String
+from sqlalchemy.orm import Mapped, mapped_column
+
+from deerflow.persistence.base import Base
+
+
+class ThreadMetaRow(Base):
+    __tablename__ = "threads_meta"
+
+    thread_id: Mapped[str] = mapped_column(String(64), primary_key=True)
+    assistant_id: Mapped[str | None] = mapped_column(String(128), index=True)
+    owner_id: Mapped[str | None] = mapped_column(String(64), index=True)
+    display_name: Mapped[str | None] = mapped_column(String(256))
+    status: Mapped[str] = mapped_column(String(20), default="idle")
+    metadata_json: Mapped[dict] = mapped_column(JSON, default=dict)
+    created_at: Mapped[datetime] = mapped_column(default=lambda: datetime.now(UTC))
+    updated_at: Mapped[datetime] = mapped_column(default=lambda: datetime.now(UTC), onupdate=lambda: datetime.now(UTC))