fix(installer): preserve secrets across re-runs

Re-running the installer after a failed build regenerated JWT_SECRET
and MARIADB_ROOT_PASS, overwriting .env with values that didn't match
the already-set MariaDB password. Now reads existing .env values and
only generates new ones on a fresh install.

server/src/app.ts

@@ -73,7 +73,7 @@ export async function buildApp() {
 
   fastify.get('/api/health', async () => ({
     status: 'ok',
-    version: '0.6.4',
+    version: '0.6.5',
     timestamp: new Date().toISOString(),
   }))