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 +1,9 @@
+--- Improve indexing for SNS worker selection queries
+
+CREATE INDEX IF NOT EXISTS idx_pbs_computations_pending_created_at
+ON pbs_computations (created_at, handle)
+WHERE is_completed = FALSE;
+
+CREATE INDEX IF NOT EXISTS idx_ciphertexts_handle_not_null
+ON ciphertexts (handle)
+WHERE ciphertext IS NOT NULL;