refactor(persistence): organize entities into per-entity directories

Restructure the persistence layer from horizontal "models/ + repositories/"
split into vertical entity-aligned directories. Each entity (thread_meta,
run, feedback) now owns its ORM model, abstract interface (where applicable),
and concrete implementations under a single directory with an aggregating
__init__.py for one-line imports.

Layout:
  persistence/thread_meta/{base,model,sql,memory}.py
  persistence/run/{model,sql}.py
  persistence/feedback/{model,sql}.py

models/__init__.py is kept as a facade so Alembic autogenerate continues to
discover all ORM tables via Base.metadata. RunEventRow remains under
models/run_event.py because its storage implementation lives in
runtime/events/store/db.py and has no matching repository directory.

The repositories/ directory is removed entirely. All call sites in
gateway/deps.py and tests are updated to import from the new entity
packages, e.g.:

    from deerflow.persistence.thread_meta import ThreadMetaRepository
    from deerflow.persistence.run import RunRepository
    from deerflow.persistence.feedback import FeedbackRepository

Full test suite passes (1690 passed, 14 skipped).

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

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

@@ -0,0 +1,51 @@
+"""Abstract interface for thread metadata storage.
+
+Implementations:
+- ThreadMetaRepository: SQL-backed (sqlite / postgres via SQLAlchemy)
+- MemoryThreadMetaStore: wraps LangGraph BaseStore (memory mode)
+"""
+
+from __future__ import annotations
+
+import abc
+
+
+class ThreadMetaStore(abc.ABC):
+    @abc.abstractmethod
+    async def create(
+        self,
+        thread_id: str,
+        *,
+        assistant_id: str | None = None,
+        owner_id: str | None = None,
+        display_name: str | None = None,
+        metadata: dict | None = None,
+    ) -> dict:
+        pass
+
+    @abc.abstractmethod
+    async def get(self, thread_id: str) -> dict | None:
+        pass
+
+    @abc.abstractmethod
+    async def search(
+        self,
+        *,
+        metadata: dict | None = None,
+        status: str | None = None,
+        limit: int = 100,
+        offset: int = 0,
+    ) -> list[dict]:
+        pass
+
+    @abc.abstractmethod
+    async def update_display_name(self, thread_id: str, display_name: str) -> None:
+        pass
+
+    @abc.abstractmethod
+    async def update_status(self, thread_id: str, status: str) -> None:
+        pass
+
+    @abc.abstractmethod
+    async def delete(self, thread_id: str) -> None:
+        pass