perf(indexers): offload sync embed_text to thread across background workers

Connector kb_sync_services (gmail, onedrive, google_calendar, jira),
streaming indexers (discord, luma, teams) and the file-processor save
path all called embed_text inside async coroutines, blocking the
background worker's event loop for the duration of the embed. Wrap each
call site in asyncio.to_thread so concurrent indexing tasks stop
serialising on the embed.

surfsense_backend/app/services/gmail/kb_sync_service.py

@@ -1,3 +1,4 @@
+import asyncio
 import logging
 from datetime import datetime
 
@@ -100,7 +101,9 @@ class GmailKBSyncService:
             else:
                 logger.warning("No LLM configured -- using fallback summary")
                 summary_content = f"Gmail Message: {subject}\n\n{indexable_content}"
-                summary_embedding = embed_text(summary_content)
+                summary_embedding = await asyncio.to_thread(
+                    embed_text, summary_content
+                )
 
             chunks = await create_document_chunks(indexable_content)
             now_str = datetime.now().strftime("%Y-%m-%d %H:%M:%S")