fix(coprocessor): db migration, improve indexing for sns worker fetching work

Author: rudy-6-4

coprocessor/fhevm-engine/db-migration/migrations/20251230155309_improve_sns_worker_select_indexing.sql

@@ -0,0 +1,7 @@
+CREATE INDEX CONCURRENTLY idx_pbs_computations_pending_created_at
+ON pbs_computations (created_at, handle)
+WHERE is_completed = FALSE;Second,
+
+CREATE INDEX CONCURRENTLY idx_ciphertexts_handle_not_null
+ON ciphertexts (handle)
+WHERE ciphertext IS NOT NULL;