fix(db): drop duplicate snapshots sandbox_id index

packages/db/migrations/20260725040700_drop_duplicate_snapshots_sandbox_id_index.sql

@@ -0,0 +1,36 @@
+-- +goose Up
+-- +goose NO TRANSACTION
+
+-- DROP only waits on in-flight lock holders; bound the wait so it neither
+-- hangs unbounded nor dies to a short session default.
+SET statement_timeout = '1h';
+
+-- Exact duplicate of snapshots_sandbox_id_unique (same single column, same
+-- order): the planner always has the unique index available, and this copy
+-- recorded 0 lifetime scans in pg_stat_user_indexes while every snapshot
+-- write paid its maintenance tax.
+DROP INDEX CONCURRENTLY IF EXISTS public.idx_snapshots_sandbox_id;
+
+-- The migrator session is reused for subsequent migrations: restore its
+-- baseline (scripts/migrator.go sets 3h per connection; RESET would clear
+-- to the unbounded server default instead).
+SET statement_timeout = '3h';
+
+-- +goose Down
+-- +goose NO TRANSACTION
+
+-- A concurrent rebuild on a table this size can run for hours, and a
+-- mid-build timeout would strand an INVALID index. Rolling back is a
+-- deliberate manual operation with an operator present — unbounded on
+-- purpose.
+SET statement_timeout = 0;
+
+-- An interrupted CONCURRENTLY build leaves an INVALID index that IF NOT
+-- EXISTS would then skip forever — clear any leftover before rebuilding.
+DROP INDEX CONCURRENTLY IF EXISTS public.idx_snapshots_sandbox_id;
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_snapshots_sandbox_id
+    ON public.snapshots (sandbox_id);
+
+-- Restore the migrator session baseline (see Up).
+SET statement_timeout = '3h';