fix: Chat search index, DMS group-share user_id, verify workflows and mail salt

- Implement CommSearchProvider: index_message(), reindex_all() with FTS + vector search
- Add migration 0035: search_tsv + embedding columns on comm_messages
- Fix DMS group-share: use current_user user_id instead of random uuid4()
- Workflows already DB-configurable (seed from onboarding.py, loaded from DB)
- Mail salt already dynamic with legacy fallback (migration 0026)

app/plugins/builtins/dms/routes.py

@@ -878,7 +878,7 @@ async def share_file(
             perm = Permission(
                 tenant_id=tenant_id,
                 file_id=fid,
-                user_id=uuid.uuid4(),  # placeholder user_id for group shares
+                user_id=uuid.UUID(current_user["user_id"]),
                 group_id=gid,
                 access_level=body.access_level,
             )