chore: bump version to 1.7.21 and update CHANGELOG and docs

server/index.js

@@ -249,7 +249,7 @@ app.use(express.json({ limit: '64kb' })); // prevent oversized JSON payloads
  *                 version:
  *                   type: string
  *                   description: sofarr version
- *                   example: "1.7.20"
+ *                   example: "1.7.21"
  */
 app.get('/health', (req, res) => {
   res.json({ status: 'ok', uptime: process.uptime(), version });