refactor(gateway): route all thread metadata access through ThreadMetaStore

Following the rename/delete bug fix in PR1, migrate the remaining direct
LangGraph Store reads/writes in the threads router and services to the
ThreadMetaStore abstraction so that the sqlite and memory backends behave
identically and the legacy dual-write paths can be removed.

Migrated endpoints (threads.py):
- create_thread: idempotency check + write now use thread_meta_repo.get/create
  instead of dual-writing the LangGraph Store and the SQL row.
- get_thread: reads from thread_meta_repo.get; the checkpoint-only fallback
  for legacy threads is preserved.
- patch_thread: replaced _store_get/_store_put with thread_meta_repo.update_metadata.
- delete_thread_data: dropped the legacy store.adelete; thread_meta_repo.delete
  already covers it.

Removed dead code (services.py):
- _upsert_thread_in_store — redundant with the immediately following
  thread_meta_repo.create() call.
- _sync_thread_title_after_run — worker.py's finally block already syncs
  the title via thread_meta_repo.update_display_name() after each run.

Removed dead code (threads.py):
- _store_get / _store_put / _store_upsert helpers (no remaining callers).
- THREADS_NS constant.
- get_store import (router no longer touches the LangGraph Store directly).

New abstract method:
- ThreadMetaStore.update_metadata(thread_id, metadata) merges metadata into
  the thread's metadata field. Implemented in both ThreadMetaRepository (SQL,
  read-modify-write inside one session) and MemoryThreadMetaStore. Three new
  unit tests cover merge / empty / nonexistent behaviour.

Net change: -134 lines. Full test suite: 1693 passed, 14 skipped.
Verified end-to-end with curl in gateway mode against sqlite backend
(create / patch / get / rename / search / delete).

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

backend/tests/test_thread_meta_repo.py

@@ -130,3 +130,28 @@ class TestThreadMetaRepository:
         repo = await _make_repo(tmp_path)
         await repo.delete("nonexistent")  # should not raise
         await _cleanup()
+
+    @pytest.mark.anyio
+    async def test_update_metadata_merges(self, tmp_path):
+        repo = await _make_repo(tmp_path)
+        await repo.create("t1", metadata={"a": 1, "b": 2})
+        await repo.update_metadata("t1", {"b": 99, "c": 3})
+        record = await repo.get("t1")
+        # Existing key preserved, overlapping key overwritten, new key added
+        assert record["metadata"] == {"a": 1, "b": 99, "c": 3}
+        await _cleanup()
+
+    @pytest.mark.anyio
+    async def test_update_metadata_on_empty(self, tmp_path):
+        repo = await _make_repo(tmp_path)
+        await repo.create("t1")
+        await repo.update_metadata("t1", {"k": "v"})
+        record = await repo.get("t1")
+        assert record["metadata"] == {"k": "v"}
+        await _cleanup()
+
+    @pytest.mark.anyio
+    async def test_update_metadata_nonexistent_is_noop(self, tmp_path):
+        repo = await _make_repo(tmp_path)
+        await repo.update_metadata("nonexistent", {"k": "v"})  # should not raise
+        await _cleanup()