fix(db): earlier autovacuum on snapshots (#3368)

Applies the same per-table autovacuum override `env_builds` already
received (20260723030000) to `public.snapshots`.

**Why this is correct:** the change is a storage-parameter update only —
it alters when autovacuum schedules a pass, never what any query reads
or writes. It takes a brief ShareUpdateExclusive lock (no rewrite, no
downtime) and is trivially revertible with `RESET`.

**What it improves:** the default trigger fires only after dead tuples
reach a fixed fraction of the table, and on a large table that means
cleanup starts far too late — meanwhile every read of this table pays
visibility checks against the accumulated dead row versions, so scan and
lookup costs creep up between passes. This table sits on hot read paths
(snapshot lookups during sandbox lifecycle operations), so keeping its
dead-tuple population small keeps those paths at their baseline cost.
Triggering earlier also makes each pass smaller and cheaper — frequent
small vacuums instead of rare huge ones.

Author: ValentaTomas

packages/db/migrations/20260724051151_snapshots_autovacuum_scale.sql

@@ -0,0 +1,22 @@
+-- +goose Up
+-- Same treatment env_builds received (20260723030000): the default autovacuum
+-- scale factors trigger a pass only after dead tuples reach a fixed fraction
+-- of the table, which on a large table defers cleanup far too long — index
+-- lookups pay visibility checks on the accumulated dead entries the whole
+-- time. Storage-parameter change only: takes a brief ShareUpdateExclusive
+-- lock, rewrites nothing, affects autovacuum scheduling alone.
+-- Inserts outnumber updates on this table, so the insert-driven threshold
+-- is lowered alongside the dead-tuple one: it keeps the visibility map
+-- fresh for the hot snapshot read paths between update-driven passes.
+ALTER TABLE public.snapshots SET (
+    autovacuum_vacuum_scale_factor = 0.02,
+    autovacuum_vacuum_insert_scale_factor = 0.02,
+    autovacuum_analyze_scale_factor = 0.01
+);
+
+-- +goose Down
+ALTER TABLE public.snapshots RESET (
+    autovacuum_vacuum_scale_factor,
+    autovacuum_vacuum_insert_scale_factor,
+    autovacuum_analyze_scale_factor
+);