chore(coprocessor): add pending-tasks idx to optimize pbs_computation backlog (#1477)

chore(coprocessor): add pending-tasks idx to optimize pbs_computations backlog

Author: goshawk-3

coprocessor/fhevm-engine/db-migration/migrations/20251205070512_add_pbs_computations_created_at_idx.sql

@@ -0,0 +1,6 @@
+-- Pending tasks index for pbs_computations table
+-- This index improves the performance of queries that fetch pending tasks
+-- based on their creation time.
+CREATE INDEX idx_pending_tasks
+    ON pbs_computations USING btree (created_at)
+    WHERE is_completed = false;