fix(worker): know you are in a worker from any thread, not only the task's

Celery records the executing task on the thread that runs it, so a thread that
task starts sees none. The embeddings client and read_document both decided
"am I in a worker?" from that alone, and from any other thread took the
web-process branch: dispatch to the worker they were running in and block on
the result. Celery refuses that get() ("Never call result.get() within a
task!"), so the embed failed and latched the 30s dispatch cooldown for every
caller after it; with joins allowed, read_document would instead wait on a
parsing queue only its own busy process serves.

Threads inside tasks are not hypothetical: per-source retrieval fans out to a
pool, so a scheduled or webhook agent searching several sources embedded from
pool threads. Graph extraction did too, which failed every chunk of a build.

in_worker() in celery_init answers for the whole process. Celery's
task_join_will_block is process-wide and set for every blocking pool (prefork,
solo, threads) -- exactly the condition under which dispatch-and-wait goes
wrong; eventlet/gevent leave it unset, so the task's own thread still counts
through current_worker_task. Verified with real workers on each blocking pool:
from a thread a task started, the old check dispatched and hit the error, the
new one embedded locally.

docsgpt/vectorstore/embeddings_delegated.py

@@ -10,8 +10,9 @@ Celery and the vector comes back. The API pays a broker round trip per query
 and no resident model.
 
 Inside a worker there is nothing to delegate to -- dispatching would queue work
-behind the task already running and wait on itself -- so a call made while a
-task is executing runs locally, on a model this process loads once and caches.
+behind the task already running and wait on itself -- so a call made anywhere in
+a worker process, including from a thread a task started, runs locally, on a
+model this process loads once and caches.
 ``DOCUMENT_PARSE_QUEUE`` exists for the same reason on the parsing side.
 
 Production deployments should point ``EMBEDDINGS_BASE_URL`` at a real embedding
@@ -79,11 +80,11 @@ def _forget(result) -> None:
 
 
 def _in_worker() -> bool:
-    """True when a Celery task is executing in this process."""
+    """True anywhere in a Celery worker process -- on any thread, not only the task's."""
     try:
-        from docsgpt.celery_init import celery
+        from docsgpt.celery_init import in_worker
 
-        return celery.current_worker_task is not None
+        return in_worker()
     except Exception:
         return False