fix: finish cross-platform test parity while keeping POSIX deployment semantics

- cutover readState: enforce 0600 mode bits only on POSIX (Windows ACLs
  govern access; chmod is a no-op there)
- backup activation: skip the read-only-handle fsync on win32; the staged
  rename-over-open-WAL tests keep running on the POSIX deployment targets
- test-fixtures: normalize fixture paths to POSIX separators before
  comparing with manifest entries

apps/api/src/infrastructure/database/database.test.ts

@@ -362,7 +362,9 @@ describe('database backup and restore', () => {
     migrateDatabase(database);
     const backupPath = join(directory, 'backup.sqlite');
     await backupDatabase(database, backupPath);
-    expect((await lstat(backupPath)).mode & 0o777).toBe(0o600);
+    // Windows keeps no POSIX mode bits; the 0600 guarantee is POSIX-only.
+    if (process.platform !== 'win32')
+      expect((await lstat(backupPath)).mode & 0o777).toBe(0o600);
     database.close();
   });