perf: split attribution_enforced_versions subquery to use indexes

The original view used a single correlated `not exists` with an `or`,
which prevented the partial unique indexes on `attributions_exemptions`
from being used. Splitting into two `not exists` clauses lets each
anti-join use its own index instead of a full sequential scan.

apps/labrinth/migrations/20260624120000_fix_attribution_enforced_versions_view.sql

@@ -0,0 +1,20 @@
+-- The original view used a single correlated `not exists` with an `or`, which
+-- prevents the partial unique indexes on `attributions_exemptions (version_id)`
+-- and `(project_id)` from being used, forcing a full sequential scan of the
+-- exemptions table for every candidate version.
+--
+-- `not exists (a or b)` is equivalent to `not exists (a) and not exists (b)`,
+-- so splitting the subquery lets each anti-join use its own index.
+create or replace view attribution_enforced_versions as
+select v.id
+from versions v
+where not exists (
+	select 1
+	from attributions_exemptions ae
+	where ae.version_id = v.id
+)
+and not exists (
+	select 1
+	from attributions_exemptions ae
+	where ae.project_id = v.mod_id
+);