Rewrite orderedUserMMRBySeason as window function to avoid 12s plan

The max(id) self-join let the query planner pick a nested-loop plan when
it misjudged a season's row count (e.g. a freshly started season whose
stats are dwarfed by older seasons), making the query take ~12s on the
current season. The window-function form references Skill once and is
plan-stable regardless of stats. Output is identical.

app/features/mmr/queries/orderedMMRBySeason.server.ts

@@ -1,24 +1,24 @@
 import { sql } from "~/db/sql";
 import type { Tables } from "~/db/tables";
 
+// Window function (latest Skill row per user) rather than a self-join against a
+// `max(id)` subquery: the self-join lets the planner pick a nested-loop plan when
+// it misjudges the season's row count (e.g. a freshly started season whose stats
+// are dwarfed by older seasons), which made this query take ~12s. This form is
+// plan-stable regardless of stats.
 const userStm = sql.prepare(/* sql */ `
-  select
-    "Skill"."ordinal",
-    "Skill"."matchesCount",
-    "Skill"."userId"
-  from
-    "Skill"
-  inner join (
-    select "userId", max("id") as "maxId"
+  select "ordinal", "matchesCount", "userId"
+  from (
+    select
+      "ordinal",
+      "matchesCount",
+      "userId",
+      row_number() over (partition by "userId" order by "id" desc) as "rn"
     from "Skill"
-    where "Skill"."season" = @season
-    group by "userId"
-  ) "Latest" on "Skill"."userId" = "Latest"."userId" and "Skill"."id" = "Latest"."maxId"
-  where
-    "Skill"."season" = @season
-    and "Skill"."userId" is not null
-  order by
-    "Skill"."ordinal" desc
+    where "season" = @season and "userId" is not null
+  )
+  where "rn" = 1
+  order by "ordinal" desc
 `);
 
 export function orderedUserMMRBySeason(season: number) {