fix: avoid MySQL thread_meta refresh races and harden roundtable bootstrap

Skip post-commit ORM refresh on threads_meta creates so concurrent MySQL
read/write-split deployments no longer fail with Could not refresh instance;
clean up partial roundtable threads on checkpoint failure, and ship related
position-roundtable/frontend build fixes.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend-web/vite.config.ts

@@ -45,7 +45,11 @@ export default defineConfig({
       },
     ],
   },
-  "base":"./",
+  // Keep the current portable-file default, while allowing container builds to
+  // emit absolute asset URLs.  The latter is required for BrowserRouter pages
+  // behind Nginx: a request to /page/... must still load /assets/... rather
+  // than /page/assets/....
+  base: process.env.VITE_ASSET_BASE ?? "./",
   build: {
     cssMinify: "lightningcss",
     target: "chrome100",