Fix: usage of postgresql -> postgres for db_type (#10120)

### What problem does this PR solve?

This PR fixes incorrect naming for PostgreSQL usage by replacing all
instances of `postgresql` with the correct `postgres` in the `db_type`
field. This resolves potential configuration errors and ensures
consistency when specifying the database type.

Also fixed handling of None for `get_queue_length`

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

Co-authored-by: cucusenok <BP-116: updated readme.md>

api/db/services/document_service.py

@@ -702,6 +702,8 @@ def queue_raptor_o_graphrag_tasks(doc, ty, priority):
 
 def get_queue_length(priority):
     group_info = REDIS_CONN.queue_info(get_svr_queue_name(priority), SVR_CONSUMER_GROUP_NAME)
+    if not group_info:
+        return 0
     return int(group_info.get("lag", 0) or 0)