Adopt sqlc for typed Postgres queries behind store entities.

db/queries/votes.sql

@@ -0,0 +1,14 @@
+-- name: GetVote :one
+SELECT value
+FROM votes
+WHERE user_id = $1 AND question_id = $2;
+
+-- name: DeleteVote :exec
+DELETE FROM votes
+WHERE user_id = $1 AND question_id = $2;
+
+-- name: UpsertVote :exec
+INSERT INTO votes (user_id, question_id, value)
+VALUES ($1, $2, $3)
+ON CONFLICT (user_id, question_id) DO UPDATE
+SET value = excluded.value;