fix: honor underscore-unused convention and make durability fsyncs cross-platform

- eslint: respect the repo's existing _-prefix convention for unused vars
- phase-one-blockers: derive projectRoot via import.meta.dirname so Windows
  checkouts stop producing C:\C:\... paths
- backup/release-evidence/cutover-orchestrator: skip POSIX-only directory
  fsync on win32 and fsync read-only handles through a writable handle

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

@@ -159,7 +159,8 @@ async function digestFileHandle(handle: FileHandle): Promise<string> {
 }
 
 async function syncFile(path: string): Promise<void> {
-  const handle = await open(path, 'r');
+  // Windows FlushFileBuffers requires a writable handle; POSIX allows fsync on 'r'.
+  const handle = await open(path, process.platform === 'win32' ? 'r+' : 'r');
   try {
     await handle.sync();
   } finally {
@@ -168,6 +169,7 @@ async function syncFile(path: string): Promise<void> {
 }
 
 async function syncDirectory(path: string): Promise<void> {
+  if (process.platform === 'win32') return; // directory fsync only exists on POSIX
   const handle = await open(path, 'r');
   try {
     await handle.sync();