appview/db: improve repo-languages query

switch from corelated query to window function. practically, this gives
us a 10x speedup for this portion of the function. we can go further by
running each subquery for GetRepos concurrently, but the potential wins
here is in the order of ~2ms.

Signed-off-by: oppiliappan <[e-mail]>

appview/db/repos.go

···
 
 	languageQuery := fmt.Sprintf(
 		`
-		select
-			repo_at, language
-		from
-			repo_languages r1
-		where
-			repo_at IN (%s)
+		select repo_at, language
+		from (
+			select
+			repo_at,
+			language,
+			row_number() over (
+				partition by repo_at
+				order by bytes desc
+			) as rn
+			from repo_languages
+			where repo_at in (%s)
 			and is_default_ref = 1
-			and id = (
-				select id
-					from repo_languages r2
-					where r2.repo_at = r1.repo_at
-					and r2.is_default_ref = 1
-					order by bytes desc
-					limit 1
-			);
+		)
+		where rn = 1
 		`,
 		inClause,
 	)