Adopt sqlc for typed Postgres queries behind store entities.

db/queries/answers.sql

@@ -0,0 +1,11 @@
+-- name: UpsertAnswer :exec
+INSERT INTO answers (question_id, author_id, body, created_at, updated_at)
+VALUES ($1, $2, $3, $4, $5)
+ON CONFLICT (question_id) DO UPDATE
+SET body = excluded.body, author_id = excluded.author_id, updated_at = excluded.updated_at;
+
+-- name: GetAnswer :one
+SELECT a.question_id, a.author_id, u.name AS author_name, a.body, a.created_at, a.updated_at
+FROM answers a
+JOIN users u ON u.id = a.author_id
+WHERE a.question_id = $1;