fix(persistence): stream hang when run_events.backend=db

DbRunEventStore._user_id_from_context() returned user.id without
coercing it to str. User.id is a Pydantic UUID, and aiosqlite cannot
bind a raw UUID object to a VARCHAR column, so the INSERT for the
initial human_message event silently rolled back and raised out of
the worker task. Because that put() sat outside the worker's try
block, the finally-clause that publishes end-of-stream never ran
and the SSE stream hung forever.

jsonl mode was unaffected because json.dumps(default=str) coerces
UUID objects transparently.

Fixes:
- db.py: coerce user.id to str at the context-read boundary (matches
  what resolve_user_id already does for the other repositories)
- worker.py: move RunJournal init + human_message put inside the try
  block so any failure flows through the finally/publish_end path
  instead of hanging the subscriber

Defense-in-depth:
- engine.py: add PRAGMA busy_timeout=5000 so checkpointer and event
  store wait for each other on the shared deerflow.db file instead
  of failing immediately under write-lock contention
- journal.py: skip fire-and-forget _flush_sync when a previous flush
  task is still in flight, to avoid piling up concurrent put_batch
  writes on the same SQLAlchemy engine during streaming; flush() now
  waits for pending tasks before draining the buffer
- database_config.py: doc-only update clarifying WAL + busy_timeout
  keep the unified deerflow.db safe for both workloads

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

backend/packages/harness/deerflow/runtime/events/store/db.py

@@ -62,9 +62,15 @@ class DbRunEventStore(RunEventStore):
         which is the expected case for background worker writes. HTTP
         request writes will have the contextvar set by auth middleware
         and get their user_id stamped automatically.
+
+        Coerces ``user.id`` to ``str`` at the boundary: ``User.id`` is
+        typed as ``UUID`` by the auth layer, but ``run_events.user_id``
+        is ``VARCHAR(64)`` and aiosqlite cannot bind a raw UUID object
+        to a VARCHAR column ("type 'UUID' is not supported") — the
+        INSERT would silently roll back and the worker would hang.
         """
         user = get_current_user()
-        return user.id if user is not None else None
+        return str(user.id) if user is not None else None
 
     async def put(self, *, thread_id, run_id, event_type, category, content="", metadata=None, created_at=None):  # noqa: D401
         """Write a single event — low-frequency path only.