feat: add job status and download endpoints

Also adds vitest.config.js with fileParallelism: false. Every DB-backed
test file wipes and reseeds the shared conversion_jobs table in
beforeEach; running test files in parallel (Vitest's default) let one
file's DELETE race another file's just-inserted row against the same
live MariaDB instance, causing intermittent cross-file failures.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

vitest.config.js

@@ -0,0 +1,7 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    fileParallelism: false,
+  },
+});