fix(db): drop duplicate snapshots sandbox_id index

[ValentaTomas]

Summary

idx_snapshots_sandbox_id is an exact duplicate of snapshots_sandbox_id_unique (same single column, same order). The unique index serves all lookups; the copy has 0 lifetime scans in pg_stat_user_indexes, while every snapshot write maintains it for nothing.

DROP INDEX CONCURRENTLY with the same timeout/INVALID-leftover handling as the recent drop-index precedent. Down rebuilds it concurrently (unbounded timeout, manual operation by design).

[cursor]

PR Summary

Low Risk
Read-path coverage stays on the existing unique constraint; the change is a concurrent index drop with established timeout and rollback handling on a large table.

Overview
Removes the redundant non-unique idx_snapshots_sandbox_id index on snapshots.sandbox_id because it mirrors snapshots_sandbox_id_unique and only adds write overhead. The migration drops it with DROP INDEX CONCURRENTLY, uses a bounded statement_timeout for the Up path and restores the migrator’s 3h session default afterward; Down can rebuild the index concurrently with unbounded timeout and clears any INVALID leftover first.

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[ValentaTomas]

closing for now, will revisit