fix(dsn-cache): replace Bun.file().exists() with stat() in validateFileMtime

Bun.file().exists() can hang on Linux CI (overlayfs/Docker environments).
Use stat() from node:fs/promises consistently with validateDirMtime.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: betegon

src/lib/db/dsn-cache.ts

@@ -253,13 +253,10 @@ async function validateFileMtime(
   cachedMtime: number
 ): Promise<boolean> {
   try {
-    const file = Bun.file(fullPath);
-    if (!(await file.exists())) {
-      return false;
-    }
-    return file.lastModified === cachedMtime;
+    const stats = await stat(fullPath);
+    return Math.floor(stats.mtimeMs) === cachedMtime;
   } catch {
-    return false;
+    return false; // File missing or inaccessible — treat as changed
   }
 }