fix(bin): close DBs in migrateDB so it flushes and exits (#7982)

`bin/migrateDB.ts` opens a source and target ueberdb2 Database, copies all
keys, then resolves without closing either connection or calling
process.exit(). Two problems with ueberdb2 6.1.x:

- 6.1.x keeps an internal keep-alive timer running until close() is called,
  so the migration process hangs forever after "Done syncing dbs" instead
  of exiting. (Pre-6.1.x it exited on its own once the work was done.)
- Target writes are buffered and only guaranteed flushed to disk on close()
  /flush(), so an operator who Ctrl-Cs the apparently-finished process could
  end up with an incomplete migration.

Close the target then the source on both the success and error paths (which
flushes buffered writes and clears the keep-alive timer) and exit with an
explicit status code, matching the pattern already used in
migrateDirtyDBtoRealDB.ts.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

bin/migrateDB.ts

@@ -74,10 +74,20 @@ const handleSync = async ()=>{
   }
 }
 
-handleSync().then(()=>{
+handleSync().then(async ()=>{
+  // Closing flushes any buffered writes to the target DB and clears
+  // ueberdb2's keep-alive timer (added in 6.1.x). Without this the migrated
+  // data may not be fully persisted and the process would hang forever
+  // instead of exiting once the sync is done.
+  await ueberdb2.close()
+  await ueberdb1.close()
   console.log("Done syncing dbs")
-}).catch(e=>{
+  process.exit(0)
+}).catch(async e=>{
   console.log(`Error syncing db ${e}`)
+  await ueberdb2.close().catch(()=>{})
+  await ueberdb1.close().catch(()=>{})
+  process.exit(1)
 })