Phase 0: fixes from review (torn files, pending blobs, exact sequences, UTF-8, storage checks)

- detect files changing during reading or between resumable steps and
  re-read them (size/mtime/ctime/inode), spike F overwrites mid-run
- drop pending blob ids when a pack or index upload fails
- dump and restore PostgreSQL sequence state exactly, bound parameters,
  validated catalog names; spike C checks all sequences exactly
- keep invalid UTF-8 byte-exact (tagged base64)
- refuse large tables without a primary key
- StorageBackend: check write/flush/close results, validate base path
- user_migration import/export: root paths, intermediate folders,
  partial writes, read errors
- spikes: never touch an existing account, own fixtures, 512M note

lib/Repository/BlobIndex.php

@@ -55,6 +55,13 @@ final class BlobIndex {
 		$this->pending[$id] = true;
 	}
 
+	/** @param list<string> $ids */
+	public function dropPending(array $ids): void {
+		foreach ($ids as $id) {
+			unset($this->pending[$id]);
+		}
+	}
+
 	/** @param list<array{id:string, offset:int, length:int, raw:int, flags:int}> $entries */
 	public function addPack(string $packId, array $entries): void {
 		$stream = fopen('php://memory', 'w+b');